Related
I have the below code:
public class Foo {
private volatile Map<String, String> map;
public Foo() {
refresh();
}
public void refresh() {
map = getData();
}
public boolean isPresent(String id) {
return map.containsKey(id);
}
public String getName(String id) {
return map.get(id);
}
private Map<String, String> getData() {
// logic
}
}
Is the above code thread safe or do I need to add synchronized or mutexes in there? If it's not thread safe, please clarify why.
Also, I've read that one should use AtomicReference instead of this, but in the source of the AtomicReference class, I can see that the field used to hold the value is volatile (along with a few convenience methods).
Is there a specific reason to use AtomicReference instead?
I've read multiple answer related to this but the concept of volatile still confuses me. Thanks in advance.
If you're not modifying the contents of map (except inside of refresh() when creating it), then there are no visibility issues in the code.
It's still possible to do isPresent(), refresh(), getName() (if no outside synchronization is used) and end up with isPresent()==true and getName()==null.
A class is "thread safe" if it does the right thing when it is used by multiple threads at the same time. There is no way to tell whether a class is thread safe unless you can say what "the right thing" means, and especially, what "the right thing when used by multiple threads" means.
What is the right thing if thread A calls foo.isPresent("X") and it returns true, and then thread B calls foo.refresh(), and then thread A calls foo.getName("X")?
If you are going to claim "thread safety", then you must be very explicit about what the caller should expect in cases like that.
Volatile is only useful in this scenario to update the value immediately. It doesn't really make the code by itself thread-safe.
But because you've stated in your comment, you only update the reference and because the reference-switch is atomic, your code will be thread-safe.(from the given code)
If I understood your question correctly and your comments - your class Foo holds a Map in which only the reference should be updated e.g. a whole new Map added instead of mutating it. If this is the premise:
It does not make any difference if you declare it as volatile or not. Every read/write operation in Java is atomic itself. You will never see a half transaction on these operations. See the JLS 17.7
17.7. Non-Atomic Treatment of double and long
For the purposes of the Java programming language memory model, a single write to a non-volatile long or double value is treated as two separate writes: one to each 32-bit half. This can result in a situation where a thread sees the first 32 bits of a 64-bit value from one write, and the second 32 bits from another write.
Writes and reads of volatile long and double values are always atomic.
Writes to and reads of references are always atomic, regardless of whether they are implemented as 32-bit or 64-bit values.
Some implementations may find it convenient to divide a single write action on a 64-bit long or double value into two write actions on adjacent 32-bit values. For efficiency's sake, this behavior is implementation-specific; an implementation of the Java Virtual Machine is free to perform writes to long and double values atomically or in two parts.
Implementations of the Java Virtual Machine are encouraged to avoid splitting 64-bit values where possible. Programmers are encouraged to declare shared 64-bit values as volatile or synchronize their programs correctly to avoid possible complications.
EDIT: Although the top statement still stands as it is - for thread safety it's necessary to add volatile to reflect the immediate update on different Threads to reflect the reference update. The behavior of a Thread is to make local copy of it while with volatile it will do a happens-before relationship in other words the Threads will have the same state of the Map.
I know there are many questions about this, but I still don't quite understand. I know what both of these keywords do, but I can't determine which to use in certain scenarios. Here are a couple of examples that I'm trying to determine which is the best to use.
Example 1:
import java.net.ServerSocket;
public class Something extends Thread {
private ServerSocket serverSocket;
public void run() {
while (true) {
if (serverSocket.isClosed()) {
...
} else { //Should this block use synchronized (serverSocket)?
//Do stuff with serverSocket
}
}
}
public ServerSocket getServerSocket() {
return serverSocket;
}
}
public class SomethingElse {
Something something = new Something();
public void doSomething() {
something.getServerSocket().close();
}
}
Example 2:
public class Server {
private int port;//Should it be volatile or the threads accessing it use synchronized (server)?
//getPort() and setPort(int) are accessed from multiple threads
public int getPort() {
return port;
}
public void setPort(int port) {
this.port = port;
}
}
Any help is greatly appreciated.
A simple answer is as follows:
synchronized can always be used to give you a thread-safe / correct solution,
volatile will probably be faster, but can only be used to give you a thread-safe / correct in limited situations.
If in doubt, use synchronized. Correctness is more important than performance.
Characterizing the situations under which volatile can be used safely involves determining whether each update operation can be performed as a single atomic update to a single volatile variable. If the operation involves accessing other (non-final) state or updating more than one shared variable, it cannot be done safely with just volatile. You also need to remember that:
updates to non-volatile long or a double may not be atomic, and
Java operators like ++ and += are not atomic.
Terminology: an operation is "atomic" if the operation either happens entirely, or it does not happen at all. The term "indivisible" is a synonym.
When we talk about atomicity, we usually mean atomicity from the perspective of an outside observer; e.g. a different thread to the one that is performing the operation. For instance, ++ is not atomic from the perspective of another thread, because that thread may be able to observe state of the field being incremented in the middle of the operation. Indeed, if the field is a long or a double, it may even be possible to observe a state that is neither the initial state or the final state!
The synchronized keyword
synchronized indicates that a variable will be shared among several threads. It's used to ensure consistency by "locking" access to the variable, so that one thread can't modify it while another is using it.
Classic Example: updating a global variable that indicates the current time
The incrementSeconds() function must be able to complete uninterrupted because, as it runs, it creates temporary inconsistencies in the value of the global variable time. Without synchronization, another function might see a time of "12:60:00" or, at the comment marked with >>>, it would see "11:00:00" when the time is really "12:00:00" because the hours haven't incremented yet.
void incrementSeconds() {
if (++time.seconds > 59) { // time might be 1:00:60
time.seconds = 0; // time is invalid here: minutes are wrong
if (++time.minutes > 59) { // time might be 1:60:00
time.minutes = 0; // >>> time is invalid here: hours are wrong
if (++time.hours > 23) { // time might be 24:00:00
time.hours = 0;
}
}
}
The volatile keyword
volatile simply tells the compiler not to make assumptions about the constant-ness of a variable, because it may change when the compiler wouldn't normally expect it. For example, the software in a digital thermostat might have a variable that indicates the temperature, and whose value is updated directly by the hardware. It may change in places that a normal variable wouldn't.
If degreesCelsius is not declared to be volatile, the compiler is free to optimize this:
void controlHeater() {
while ((degreesCelsius * 9.0/5.0 + 32) < COMFY_TEMP_IN_FAHRENHEIT) {
setHeater(ON);
sleep(10);
}
}
into this:
void controlHeater() {
float tempInFahrenheit = degreesCelsius * 9.0/5.0 + 32;
while (tempInFahrenheit < COMFY_TEMP_IN_FAHRENHEIT) {
setHeater(ON);
sleep(10);
}
}
By declaring degreesCelsius to be volatile, you're telling the compiler that it has to check its value each time it runs through the loop.
Summary
In short, synchronized lets you control access to a variable, so you can guarantee that updates are atomic (that is, a set of changes will be applied as a unit; no other thread can access the variable when it's half-updated). You can use it to ensure consistency of your data. On the other hand, volatile is an admission that the contents of a variable are beyond your control, so the code must assume it can change at any time.
There is insufficient information in your post to determine what is going on, which is why all the advice you are getting is general information about volatile and synchronized.
So, here's my general advice:
During the cycle of writing-compiling-running a program, there are two optimization points:
at compile time, when the compiler might try to reorder instructions or optimize data caching.
at runtime, when the CPU has its own optimizations, like caching and out-of-order execution.
All this means that instructions will most likely not execute in the order that you wrote them, regardless if this order must be maintained in order to ensure program correctness in a multithreaded environment. A classic example you will often find in the literature is this:
class ThreadTask implements Runnable {
private boolean stop = false;
private boolean work;
public void run() {
while(!stop) {
work = !work; // simulate some work
}
}
public void stopWork() {
stop = true; // signal thread to stop
}
public static void main(String[] args) {
ThreadTask task = new ThreadTask();
Thread t = new Thread(task);
t.start();
Thread.sleep(1000);
task.stopWork();
t.join();
}
}
Depending on compiler optimizations and CPU architecture, the above code may never terminate on a multi-processor system. This is because the value of stop will be cached in a register of the CPU running thread t, such that the thread will never again read the value from main memory, even thought the main thread has updated it in the meantime.
To combat this kind of situation, memory fences were introduced. These are special instructions that do not allow regular instructions before the fence to be reordered with instructions after the fence. One such mechanism is the volatile keyword. Variables marked volatile are not optimized by the compiler/CPU and will always be written/read directly to/from main memory. In short, volatile ensures visibility of a variable's value across CPU cores.
Visibility is important, but should not be confused with atomicity. Two threads incrementing the same shared variable may produce inconsistent results even though the variable is declared volatile. This is due to the fact that on some systems the increment is actually translated into a sequence of assembler instructions that can be interrupted at any point. For such cases, critical sections such as the synchronized keyword need to be used. This means that only a single thread can access the code enclosed in the synchronized block. Other common uses of critical sections are atomic updates to a shared collection, when usually iterating over a collection while another thread is adding/removing items will cause an exception to be thrown.
Finally two interesting points:
synchronized and a few other constructs such as Thread.join will introduce memory fences implicitly. Hence, incrementing a variable inside a synchronized block does not require the variable to also be volatile, assuming that's the only place it's being read/written.
For simple updates such as value swap, increment, decrement, you can use non-blocking atomic methods like the ones found in AtomicInteger, AtomicLong, etc. These are much faster than synchronized because they do not trigger a context switch in case the lock is already taken by another thread. They also introduce memory fences when used.
Note: In your first example, the field serverSocket is actually never initialized in the code you show.
Regarding synchronization, it depends on whether or not the ServerSocket class is thread safe. (I assume it is, but I have never used it.) If it is, you don't need to synchronize around it.
In the second example, int variables can be atomically updated so volatile may suffice.
volatile solves “visibility” problem across CPU cores. Therefore, value from local registers is flushed and synced with RAM. However, if we need consistent value and atomic op, we need a mechanism to defend the critical data. That can be achieved by either synchronized block or explicit lock.
I have a problem with limiting concurrent access to a method. I have a method MyService that can be called from many places at many times. This method must return a String, that should be updated according to some rules. For this, I have an updatedString class. Before getting the String, it makes sure that the String is updated, if not, it updates it. Many threads could read the String at the same time but ONLY ONE should renew the String at the same time if it is out of date.
public final class updatedString {
private static final String UPstring;
private static final Object lock = new Object();
public static String getUpdatedString(){
synchronized(lock){
if(stringNeedRenewal()){
renewString();
}
}
return getString();
}
...
This works fine. If I have 7 threads getting the String, it guarantees that, if necessary, ONLY one thread is updating the String.
My question is, is it a good idea to have all this static? Why if not? Is it fast? Is there a better way to do this?
I have read posts like this:
What Cases Require Synchronized Method Access in Java? which suggests that static mutable variables are not a good idea, and static classes either. But I cannot see any dead-lock in the code or a better valid solution. Only that some threads will have to wait until the String is updated (if necessary) or wait for other thread to leave the synchronized block (which causes a small delay).
If the method is not static, then I have a problem because this will not work since the synchronized method acts only for the current instance that the thread is using. Synchronized the method does not work either, it seems that the lock instance-specific and not class-specific.
The other solution could be to have a Singleton that avoids creating more than one instance and then use a single synchronized not-static class, but I do not like this solution too much.
Additional information:
stringNeedRenewal() is not too expensive although it has to read from a database. renewString() on the contrary is very expensive, and has to read from several tables on the database to finally come to an answer. The String needs arbitrary renewal, but this does not happen very often (from once per hour to once per week).
#forsvarir made me think... and I think he/she was right. return getString(); MUST be inside the synchronized method. At a first sight it looks as if it can be out of it so threads will be able to read it concurrently, but what happens if a thread stops running WHILE calling getString() and other thread partially execute renewString()? We could have this situation (assuming a single processor):
THREAD 1 starts getString(). The OS
starts copying into memory the bytes
to be returned.
THREAD 1 is stopped by the OS before finishing the copy.
THREAD 2 enters the synchronized
block and starts renewString(),
changing the original String in
memory.
THREAD 1 gets control back
and finish getString using a
corrupted String!! So it copied one
part from the old string and another
from the new one.
Having the read inside the synchronized block can make everything very slow, since threads could only access this one by one.
As #Jeremy Heiler pointed out, this is an abstract problem of a cache. If the cache is old, renew it. If not, use it. It is better more clear to picture the problem like this instead of a single String (or imagine that there are 2 strings instead of one). So what happens if someone is reading at the same time as someone is modifying the cache?
First of all, you can remove the lock and the synchronized block and simply use:
public static synchronized String getUpdatedString(){
if(stringNeedRenewal()){
renewString();
}
return getString();
}
this synchronizes on the UpdatedString.class object.
Another thing you can do is used double-checked locking to prevent unnecessary waiting. Declare the string to be volatile and:
public static String getUpdatedString(){
if(stringNeedRenewal()){
synchronized(lock) {
if(stringNeedRenewal()){
renewString();
}
}
}
return getString();
}
Then, whether to use static or not - it seems it should be static, since you want to invoke it without any particular instance.
I would suggest looking into a ReentrantReadWriteLock. (Whether or not it is performant is up to you to decide.) This way you can have many read operations occur simultaneously.
Here is the example from the documentation:
class CachedData {
Object data;
volatile boolean cacheValid;
ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
void processCachedData() {
rwl.readLock().lock();
if (!cacheValid) {
// Must release read lock before acquiring write lock
rwl.readLock().unlock();
rwl.writeLock().lock();
// Recheck state because another thread might have acquired
// write lock and changed state before we did.
if (!cacheValid) {
data = ...
cacheValid = true;
}
// Downgrade by acquiring read lock before releasing write lock
rwl.readLock().lock();
rwl.writeLock().unlock(); // Unlock write, still hold read
}
use(data);
rwl.readLock().unlock();
}
}
This isn't exactly what you're after, and I'm not a Java specialist, so take this with a pinch of salt :)
Perhaps the code sample you've provided is contrived, but if not, I'm unclear what the purpose of the class is. You only want one thread to update the string to it's new value. Why? Is it to save effort (because you'd rather use the processor cycles on something else)? Is it to maintain consistentcy (once a certain point is reached, the string must be updated)?
How long is the cycle between required updates?
Looking at your code...
public final class updatedString {
private static final String UPstring;
private static final Object lock = new Object();
public static String getUpdatedString(){
synchronized(lock){
// One thread is in this block at a time
if(stringNeedRenewal()){
renewString(); // This updates the shared string?
}
}
// At this point, you're calling out to a method. I don't know what the
// method does, I'm assuming it just returns UPstring, but at this point,
// you're no longer synchronized. The string actually returned may or may
// not be the same one that was present when the thread went through the
// synchronized section hence the question, what is the purpose of the
// synchronization...
return getString(); // This returns the shared string?
}
The right locking / optimizations depend upon the reason that you're putting them in place, the likelyhood of a write being required and as Paulo has said, the cost of the operations involved.
For some situations where writes are rare, and obviously depending upon what renewString does, it may be desirable to use an optimistic write approach. Where each thread checks if a refresh is required, proceeds to perform the update on a local and then only at the end, assigns the value across to the field being read (you need to track the age of your updates if you follow this approach). This would be faster for reading, since the check for 'does the string need renewed' can be performed outside of the synchronised section. Various other approaches could be used, depending upon the individual scenario...
as long as you lock is static, everything else doesn't have to be, and things will work just as they do now
In Java, the idiomatic way to declare critical sections in the code is the following:
private void doSomething() {
// thread-safe code
synchronized(this) {
// thread-unsafe code
}
// thread-safe code
}
Almost all blocks synchronize on this, but is there a particular reason for this? Are there other possibilities? Are there any best practices on what object to synchronize on? (such as private instances of Object?)
As earlier answerers have noted, it is best practice to synchronize on an object of limited scope (in other words, pick the most restrictive scope you can get away with, and use that.) In particular, synchronizing on this is a bad idea, unless you intend to allow the users of your class to gain the lock.
A particularly ugly case arises, though, if you choose to synchronize on a java.lang.String. Strings can be (and in practice almost always are) interned. That means that each string of equal content - in the ENTIRE JVM - turns out to be the same string behind the scenes. That means that if you synchronize on any String, another (completely disparate) code section that also locks on a String with the same content, will actually lock your code as well.
I was once troubleshooting a deadlock in a production system and (very painfully) tracked the deadlock to two completely disparate open source packages that each synchronized on an instance of String whose contents were both "LOCK".
First, note that the following code snippets are identical.
public void foo() {
synchronized (this) {
// do something thread-safe
}
}
and:
public synchronized void foo() {
// do something thread-safe
}
do exactly the same thing. No preference for either one of them except for code readability and style.
When you do synchronize methods or blocks of code, it's important to know why you are doing such a thing, and what object exactly you are locking, and for what purpose.
Also note that there are situations in which you will want to client-side synchronize blocks of code in which the monitor you are asking for (i.e. the synchronized object) is not necessarily this, like in this example :
Vector v = getSomeGlobalVector();
synchronized (v) {
// some thread-safe operation on the vector
}
I suggest you get more knowledge about concurrent programming, it will serve you a great deal once you know exactly what's happening behind the scenes. You should check out Concurrent Programming in Java, a great book on the subject. If you want a quick dive-in to the subject, check out Java Concurrency # Sun
I try to avoid synchronizing on this because that would allow everybody from the outside who had a reference to that object to block my synchronization. Instead, I create a local synchronization object:
public class Foo {
private final Object syncObject = new Object();
…
}
Now I can use that object for synchronization without fear of anybody “stealing” the lock.
Just to highlight that there are also ReadWriteLocks available in Java, found as java.util.concurrent.locks.ReadWriteLock.
In most of my usage, I seperate my locking as 'for reading' and 'for updates'. If you simply use a synchronized keyword, all reads to the same method/code block will be 'queued'. Only one thread can access the block at one time.
In most cases, you never have to worry about concurrency issues if you are simply doing reading. It is when you are doing writing that you worry about concurrent updates (resulting in lost of data), or reading during a write (partial updates), that you have to worry about.
Therefore a read/write lock makes more sense to me during multi-threaded programming.
You'll want to synchronize on an object that can serve as a Mutex. If the current instance (the this reference) is suitable (not a Singleton, for instance), you may use it, as in Java any Object may serve as the Mutex.
In other occasions, you may want to share a Mutex between several classes, if instances of these classes may all need access to the same resources.
It depends a lot on the environment you're working in and the type of system you're building. In most Java EE applications I've seen, there's actually no real need for synchronization...
Personally, I think the answers which insist that it is never or only rarely correct to sync on this are misguided. I think it depends on your API. If your class is a threadsafe implementation and you so document it, then you should use this. If the synchronization is not to make each instance of the class as a whole threadsafe in the invocation of it's public methods, then you should use a private internal object. Reusable library components often fall into the former category - you must think carefully before you disallow the user to wrap your API in external synchronization.
In the former case, using this allows multiple methods to be invoked in an atomic manner. One example is PrintWriter, where you may want to output multiple lines (say a stack trace to the console/logger) and guarantee they appear together - in this case the fact that it hides the sync object internally is a real pain. Another such example are the synchronized collection wrappers - there you must synchronize on the collection object itself in order to iterate; since iteration consists of multiple method invocations you cannot protect it totally internally.
In the latter case, I use a plain object:
private Object mutex=new Object();
However, having seen many JVM dumps and stack traces that say a lock is "an instance of java.lang.Object()" I have to say that using an inner class might often be more helpful, as others have suggested.
Anyway, that's my two bits worth.
Edit: One other thing, when synchronizing on this I prefer to sync the methods, and keep the methods very granular. I think it's clearer and more concise.
Synchronization in Java often involves synchronizing operations on the same instance. Synchronizing on this then is very idiomatic since this is a shared reference that is automatically available between different instance methods (or sections of) in a class.
Using another reference specifically for locking, by declaring and initializing a private field Object lock = new Object() for example, is something I never needed or used. I think it is only useful when you need external synchronization on two or more unsynchronized resources inside an object, although I would always try to refactor such a situation into a simpler form.
Anyway, implicit (synchronized method) or explicit synchronized(this) is used a lot, also in the Java libraries. It is a good idiom and, if applicable, should always be your first choice.
On what you synchronize depends on what other threads that might potentially get into conflict with this method call can synchronize.
If this is an object that is used by only one thread and we are accessing a mutable object which is shared between threads, a good candidate is to synchronize over that object - synchronizing on this has no point since another thread that modifies that shared object might not even know this, but does know that object.
On the other hand synchronizing over this makes sense if many threads call methods of this object at the same time, for instance if we are in a singleton.
Note that a syncronized method is often not the best option, since we hold a lock the whole time the method runs. If it contains timeconsuming but thread safe parts, and a not so time consuming thread-unsafe part, synchronizing over the method is very wrong.
Almost all blocks synchronize on this, but is there a particular reason for this? Are there other possibilities?
This declaration synchronizes entire method.
private synchronized void doSomething() {
This declaration synchronized a part of code block instead of entire method.
private void doSomething() {
// thread-safe code
synchronized(this) {
// thread-unsafe code
}
// thread-safe code
}
From oracle documentation page
making these methods synchronized has two effects:
First, it is not possible for two invocations of synchronized methods on the same object to interleave. When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.
Are there other possibilities? Are there any best practices on what object to synchronize on? (such as private instances of Object?)
There are many possibilities and alternatives to synchronization. You can make your code thread safe by using high level concurrency APIs( available since JDK 1.5 release)
Lock objects
Executors
Concurrent collections
Atomic variables
ThreadLocalRandom
Refer to below SE questions for more details:
Synchronization vs Lock
Avoid synchronized(this) in Java?
the Best Practices is to create an object solely to provide the lock:
private final Object lock = new Object();
private void doSomething() {
// thread-safe code
synchronized(lock) {
// thread-unsafe code
}
// thread-safe code
}
By doing this you are safe, that no calling code can ever deadlock your method by an unintentional synchronized(yourObject) line.
(Credits to #jared and #yuval-adam who explained this in more details above.)
My guess is that the popularity of using this in tutorials came from early Sun javadoc: https://docs.oracle.com/javase/tutorial/essential/concurrency/locksync.html
Synchronization includes 3 parts: Atomicity, Visibility and Ordering
Synchronized block is very coarse level of synchronization. It enforces visibility and ordering just as what you expected. But for atomicity, it does not provide much protection. Atomicity requires global knowledge of the program rather than local knowledge. (And that makes multi-threading programming very hard)
Let's say we have a class Account having method deposit and withdraw. They are both synchronized based on a private lock like this:
class Account {
private Object lock = new Object();
void withdraw(int amount) {
synchronized(lock) {
// ...
}
}
void deposit(int amount) {
synchronized(lock) {
// ...
}
}
}
Considering we need to implement a higher-level class which handles transfer, like this:
class AccountManager {
void transfer(Account fromAcc, Account toAcc, int amount) {
if (fromAcc.getBalance() > amount) {
fromAcc.setBalance(fromAcc.getBalance() - amount);
toAcc.setBalance(toAcc.getBalance + amount);
}
}
}
Assuming we have 2 accounts now,
Account john;
Account marry;
If the Account.deposit() and Account.withdraw() are locked with internal lock only. That will cause problem when we have 2 threads working:
// Some thread
void threadA() {
john.withdraw(500);
}
// Another thread
void threadB() {
accountManager.transfer(john, marry, 100);
}
Because it is possible for both threadA and threadB run at the same time. And thread B finishes the conditional check, thread A withdraws, and thread B withdraws again. This means we can withdraw $100 from John even if his account has no enough money. This will break atomicity.
You may propose that: why not adding withdraw() and deposit() to AccountManager then? But under this proposal, we need to create a multi-thread safe Map which maps from different accounts to their locks. We need to delete the lock after execution (otherwise will leak memory). And we also need to ensure no other one accesses the Account.withdraw() directly. This will introduce a lots of subtle bugs.
The correct and most idiomatic way is to expose the lock in the Account. And let the AccountManager to use the lock. But in this case, why not just use the object itself then?
class Account {
synchronized void withdraw(int amount) {
// ...
}
synchronized void deposit(int amount) {
// ...
}
}
class AccountManager {
void transfer(Account fromAcc, Account toAcc, int amount) {
// Ensure locking order to prevent deadlock
Account firstLock = fromAcc.hashCode() < toAcc.hashCode() ? fromAcc : toAcc;
Account secondLock = fromAcc.hashCode() < toAcc.hashCode() ? toAcc : fromAcc;
synchronized(firstLock) {
synchronized(secondLock) {
if (fromAcc.getBalance() > amount) {
fromAcc.setBalance(fromAcc.getBalance() - amount);
toAcc.setBalance(toAcc.getBalance + amount);
}
}
}
}
}
To conclude in simple English, private lock does not work for slightly more complicated multi-threaded program.
Whenever a question pops up on SO about Java synchronization, some people are very eager to point out that synchronized(this) should be avoided. Instead, they claim, a lock on a private reference is to be preferred.
Some of the given reasons are:
some evil code may steal your lock (very popular this one, also has an "accidentally" variant)
all synchronized methods within the same class use the exact same lock, which reduces throughput
you are (unnecessarily) exposing too much information
Other people, including me, argue that synchronized(this) is an idiom that is used a lot (also in Java libraries), is safe and well understood. It should not be avoided because you have a bug and you don't have a clue of what is going on in your multithreaded program. In other words: if it is applicable, then use it.
I am interested in seeing some real-world examples (no foobar stuff) where avoiding a lock on this is preferable when synchronized(this) would also do the job.
Therefore: should you always avoid synchronized(this) and replace it with a lock on a private reference?
Some further info (updated as answers are given):
we are talking about instance synchronization
both implicit (synchronized methods) and explicit form of synchronized(this) are considered
if you quote Bloch or other authorities on the subject, don't leave out the parts you don't like (e.g. Effective Java, item on Thread Safety: Typically it is the lock on the instance itself, but there are exceptions.)
if you need granularity in your locking other than synchronized(this) provides, then synchronized(this) is not applicable so that's not the issue
I'll cover each point separately.
Some evil code may steal your lock (very popular this one, also has an
"accidentally" variant)
I'm more worried about accidentally. What it amounts to is that this use of this is part of your class' exposed interface, and should be documented. Sometimes the ability of other code to use your lock is desired. This is true of things like Collections.synchronizedMap (see the javadoc).
All synchronized methods within the same class use the exact same
lock, which reduces throughput
This is overly simplistic thinking; just getting rid of synchronized(this) won't solve the problem. Proper synchronization for throughput will take more thought.
You are (unnecessarily) exposing too much information
This is a variant of #1. Use of synchronized(this) is part of your interface. If you don't want/need this exposed, don't do it.
Well, firstly it should be pointed out that:
public void blah() {
synchronized (this) {
// do stuff
}
}
is semantically equivalent to:
public synchronized void blah() {
// do stuff
}
which is one reason not to use synchronized(this). You might argue that you can do stuff around the synchronized(this) block. The usual reason is to try and avoid having to do the synchronized check at all, which leads to all sorts of concurrency problems, specifically the double checked-locking problem, which just goes to show how difficult it can be to make a relatively simple check threadsafe.
A private lock is a defensive mechanism, which is never a bad idea.
Also, as you alluded to, private locks can control granularity. One set of operations on an object might be totally unrelated to another but synchronized(this) will mutually exclude access to all of them.
synchronized(this) just really doesn't give you anything.
While you are using synchronized(this) you are using the class instance as a lock itself. This means that while lock is acquired by thread 1, the thread 2 should wait.
Suppose the following code:
public void method1() {
// do something ...
synchronized(this) {
a ++;
}
// ................
}
public void method2() {
// do something ...
synchronized(this) {
b ++;
}
// ................
}
Method 1 modifying the variable a and method 2 modifying the variable b, the concurrent modification of the same variable by two threads should be avoided and it is. BUT while thread1 modifying a and thread2 modifying b it can be performed without any race condition.
Unfortunately, the above code will not allow this since we are using the same reference for a lock; This means that threads even if they are not in a race condition should wait and obviously the code sacrifices concurrency of the program.
The solution is to use 2 different locks for two different variables:
public class Test {
private Object lockA = new Object();
private Object lockB = new Object();
public void method1() {
// do something ...
synchronized(lockA) {
a ++;
}
// ................
}
public void method2() {
// do something ...
synchronized(lockB) {
b ++;
}
// ................
}
}
The above example uses more fine grained locks (2 locks instead one (lockA and lockB for variables a and b respectively) and as a result allows better concurrency, on the other hand it became more complex than the first example ...
While I agree about not adhering blindly to dogmatic rules, does the "lock stealing" scenario seem so eccentric to you? A thread could indeed acquire the lock on your object "externally"(synchronized(theObject) {...}), blocking other threads waiting on synchronized instance methods.
If you don't believe in malicious code, consider that this code could come from third parties (for instance if you develop some sort of application server).
The "accidental" version seems less likely, but as they say, "make something idiot-proof and someone will invent a better idiot".
So I agree with the it-depends-on-what-the-class-does school of thought.
Edit following eljenso's first 3 comments:
I've never experienced the lock stealing problem but here is an imaginary scenario:
Let's say your system is a servlet container, and the object we're considering is the ServletContext implementation. Its getAttribute method must be thread-safe, as context attributes are shared data; so you declare it as synchronized. Let's also imagine that you provide a public hosting service based on your container implementation.
I'm your customer and deploy my "good" servlet on your site. It happens that my code contains a call to getAttribute.
A hacker, disguised as another customer, deploys his malicious servlet on your site. It contains the following code in the init method:
synchronized (this.getServletConfig().getServletContext()) {
while (true) {}
}
Assuming we share the same servlet context (allowed by the spec as long as the two servlets are on the same virtual host), my call on getAttribute is locked forever. The hacker has achieved a DoS on my servlet.
This attack is not possible if getAttribute is synchronized on a private lock, because 3rd-party code cannot acquire this lock.
I admit that the example is contrived and an oversimplistic view of how a servlet container works, but IMHO it proves the point.
So I would make my design choice based on security consideration: will I have complete control over the code that has access to the instances? What would be the consequence of a thread's holding a lock on an instance indefinitely?
It depends on the situation.
If There is only one sharing entity or more than one.
See full working example here
A small introduction.
Threads and shareable entities
It is possible for multiple threads to access same entity, for eg multiple connectionThreads sharing a single messageQueue. Since the threads run concurrently there may be a chance of overriding one's data by another which may be a messed up situation.
So we need some way to ensure that shareable entity is accessed only by one thread at a time. (CONCURRENCY).
Synchronized block
synchronized() block is a way to ensure concurrent access of shareable entity.
First, a small analogy
Suppose There are two-person P1, P2 (threads) a Washbasin (shareable entity) inside a washroom and there is a door (lock).
Now we want one person to use washbasin at a time.
An approach is to lock the door by P1 when the door is locked P2 waits until p1 completes his work
P1 unlocks the door
then only p1 can use washbasin.
syntax.
synchronized(this)
{
SHARED_ENTITY.....
}
"this" provided the intrinsic lock associated with the class (Java developer designed Object class in such a way that each object can work as monitor).
Above approach works fine when there are only one shared entity and multiple threads (1: N).
N shareable entities-M threads
Now think of a situation when there is two washbasin inside a washroom and only one door. If we are using the previous approach, only p1 can use one washbasin at a time while p2 will wait outside. It is wastage of resource as no one is using B2 (washbasin).
A wiser approach would be to create a smaller room inside washroom and provide them one door per washbasin. In this way, P1 can access B1 and P2 can access B2 and vice-versa.
washbasin1;
washbasin2;
Object lock1=new Object();
Object lock2=new Object();
synchronized(lock1)
{
washbasin1;
}
synchronized(lock2)
{
washbasin2;
}
See more on Threads----> here
There seems a different consensus in the C# and Java camps on this. The majority of Java code I have seen uses:
// apply mutex to this instance
synchronized(this) {
// do work here
}
whereas the majority of C# code opts for the arguably safer:
// instance level lock object
private readonly object _syncObj = new object();
...
// apply mutex to private instance level field (a System.Object usually)
lock(_syncObj)
{
// do work here
}
The C# idiom is certainly safer. As mentioned previously, no malicious / accidental access to the lock can be made from outside the instance. Java code has this risk too, but it seems that the Java community has gravitated over time to the slightly less safe, but slightly more terse version.
That's not meant as a dig against Java, just a reflection of my experience working on both languages.
Make your data immutable if it is possible ( final variables)
If you can't avoid mutation of shared data across multiple threads, use high level programming constructs [e.g. granular Lock API ]
A Lock provides exclusive access to a shared resource: only one thread at a time can acquire the lock and all access to the shared resource requires that the lock be acquired first.
Sample code to use ReentrantLock which implements Lock interface
class X {
private final ReentrantLock lock = new ReentrantLock();
// ...
public void m() {
lock.lock(); // block until condition holds
try {
// ... method body
} finally {
lock.unlock()
}
}
}
Advantages of Lock over Synchronized(this)
The use of synchronized methods or statements forces all lock acquisition and release to occur in a block-structured way.
Lock implementations provide additional functionality over the use of synchronized methods and statements by providing
A non-blocking attempt to acquire a lock (tryLock())
An attempt to acquire the lock that can be interrupted (lockInterruptibly())
An attempt to acquire the lock that can timeout (tryLock(long, TimeUnit)).
A Lock class can also provide behavior and semantics that is quite different from that of the implicit monitor lock, such as
guaranteed ordering
non-re entrant usage
Deadlock detection
Have a look at this SE question regarding various type of Locks:
Synchronization vs Lock
You can achieve thread safety by using advanced concurrency API instead of Synchronied blocks. This documentation page provides good programming constructs to achieve thread safety.
Lock Objects support locking idioms that simplify many concurrent applications.
Executors define a high-level API for launching and managing threads. Executor implementations provided by java.util.concurrent provide thread pool management suitable for large-scale applications.
Concurrent Collections make it easier to manage large collections of data, and can greatly reduce the need for synchronization.
Atomic Variables have features that minimize synchronization and help avoid memory consistency errors.
ThreadLocalRandom (in JDK 7) provides efficient generation of pseudorandom numbers from multiple threads.
Refer to java.util.concurrent and java.util.concurrent.atomic packages too for other programming constructs.
The java.util.concurrent package has vastly reduced the complexity of my thread safe code. I only have anecdotal evidence to go on, but most work I have seen with synchronized(x) appears to be re-implementing a Lock, Semaphore, or Latch, but using the lower-level monitors.
With this in mind, synchronizing using any of these mechanisms is analogous to synchronizing on an internal object, rather than leaking a lock. This is beneficial in that you have absolute certainty that you control the entry into the monitor by two or more threads.
If you've decided that:
the thing you need to do is lock on
the current object; and
you want to
lock it with granularity smaller than
a whole method;
then I don't see the a taboo over synchronizezd(this).
Some people deliberately use synchronized(this) (instead of marking the method synchronized) inside the whole contents of a method because they think it's "clearer to the reader" which object is actually being synchronized on. So long as people are making an informed choice (e.g. understand that by doing so they're actually inserting extra bytecodes into the method and this could have a knock-on effect on potential optimisations), I don't particularly see a problem with this. You should always document the concurrent behaviour of your program, so I don't see the "'synchronized' publishes the behaviour" argument as being so compelling.
As to the question of which object's lock you should use, I think there's nothing wrong with synchronizing on the current object if this would be expected by the logic of what you're doing and how your class would typically be used. For example, with a collection, the object that you would logically expect to lock is generally the collection itself.
I think there is a good explanation on why each of these are vital techniques under your belt in a book called Java Concurrency In Practice by Brian Goetz. He makes one point very clear - you must use the same lock "EVERYWHERE" to protect the state of your object. Synchronised method and synchronising on an object often go hand in hand. E.g. Vector synchronises all its methods. If you have a handle to a vector object and are going to do "put if absent" then merely Vector synchronising its own individual methods isn't going to protect you from corruption of state. You need to synchronise using synchronised (vectorHandle). This will result in the SAME lock being acquired by every thread which has a handle to the vector and will protect overall state of the vector. This is called client side locking. We do know as a matter of fact vector does synchronised (this) / synchronises all its methods and hence synchronising on the object vectorHandle will result in proper synchronisation of vector objects state. Its foolish to believe that you are thread safe just because you are using a thread safe collection. This is precisely the reason ConcurrentHashMap explicitly introduced putIfAbsent method - to make such operations atomic.
In summary
Synchronising at method level allows client side locking.
If you have a private lock object - it makes client side locking impossible. This is fine if you know that your class doesn't have "put if absent" type of functionality.
If you are designing a library - then synchronising on this or synchronising the method is often wiser. Because you are rarely in a position to decide how your class is going to be used.
Had Vector used a private lock object - it would have been impossible to get "put if absent" right. The client code will never gain a handle to the private lock thus breaking the fundamental rule of using the EXACT SAME LOCK to protect its state.
Synchronising on this or synchronised methods do have a problem as others have pointed out - someone could get a lock and never release it. All other threads would keep waiting for the lock to be released.
So know what you are doing and adopt the one that's correct.
Someone argued that having a private lock object gives you better granularity - e.g. if two operations are unrelated - they could be guarded by different locks resulting in better throughput. But this i think is design smell and not code smell - if two operations are completely unrelated why are they part of the SAME class? Why should a class club unrelated functionalities at all? May be a utility class? Hmmmm - some util providing string manipulation and calendar date formatting through the same instance?? ... doesn't make any sense to me at least!!
No, you shouldn't always. However, I tend to avoid it when there are multiple concerns on a particular object that only need to be threadsafe in respect to themselves. For example, you might have a mutable data object that has "label" and "parent" fields; these need to be threadsafe, but changing one need not block the other from being written/read. (In practice I would avoid this by declaring the fields volatile and/or using java.util.concurrent's AtomicFoo wrappers).
Synchronization in general is a bit clumsy, as it slaps a big lock down rather than thinking exactly how threads might be allowed to work around each other. Using synchronized(this) is even clumsier and anti-social, as it's saying "no-one may change anything on this class while I hold the lock". How often do you actually need to do that?
I would much rather have more granular locks; even if you do want to stop everything from changing (perhaps you're serialising the object), you can just acquire all of the locks to achieve the same thing, plus it's more explicit that way. When you use synchronized(this), it's not clear exactly why you're synchronizing, or what the side effects might be. If you use synchronized(labelMonitor), or even better labelLock.getWriteLock().lock(), it's clear what you are doing and what the effects of your critical section are limited to.
Short answer: You have to understand the difference and make choice depending on the code.
Long answer: In general I would rather try to avoid synchronize(this) to reduce contention but private locks add complexity you have to be aware of. So use the right synchronization for the right job. If you are not so experienced with multi-threaded programming I would rather stick to instance locking and read up on this topic. (That said: just using synchronize(this) does not automatically make your class fully thread-safe.) This is a not an easy topic but once you get used to it, the answer whether to use synchronize(this) or not comes naturally.
A lock is used for either visibility or for protecting some data from concurrent modification which may lead to race.
When you need to just make primitive type operations to be atomic there are available options like AtomicInteger and the likes.
But suppose you have two integers which are related to each other like x and y co-ordinates, which are related to each other and should be changed in an atomic manner. Then you would protect them using a same lock.
A lock should only protect the state that is related to each other. No less and no more. If you use synchronized(this) in each method then even if the state of the class is unrelated all the threads will face contention even if updating unrelated state.
class Point{
private int x;
private int y;
public Point(int x, int y){
this.x = x;
this.y = y;
}
//mutating methods should be guarded by same lock
public synchronized void changeCoordinates(int x, int y){
this.x = x;
this.y = y;
}
}
In the above example I have only one method which mutates both x and y and not two different methods as x and y are related and if I had given two different methods for mutating x and y separately then it would not have been thread safe.
This example is just to demonstrate and not necessarily the way it should be implemented. The best way to do it would be to make it IMMUTABLE.
Now in opposition to Point example, there is an example of TwoCounters already provided by #Andreas where the state which is being protected by two different locks as the state is unrelated to each other.
The process of using different locks to protect unrelated states is called Lock Striping or Lock Splitting
The reason not to synchronize on this is that sometimes you need more than one lock (the second lock often gets removed after some additional thinking, but you still need it in the intermediate state). If you lock on this, you always have to remember which one of the two locks is this; if you lock on a private Object, the variable name tells you that.
From the reader's viewpoint, if you see locking on this, you always have to answer the two questions:
what kind of access is protected by this?
is one lock really enough, didn't someone introduce a bug?
An example:
class BadObject {
private Something mStuff;
synchronized setStuff(Something stuff) {
mStuff = stuff;
}
synchronized getStuff(Something stuff) {
return mStuff;
}
private MyListener myListener = new MyListener() {
public void onMyEvent(...) {
setStuff(...);
}
}
synchronized void longOperation(MyListener l) {
...
l.onMyEvent(...);
...
}
}
If two threads begin longOperation() on two different instances of BadObject, they acquire
their locks; when it's time to invoke l.onMyEvent(...), we have a deadlock because neither of the threads may acquire the other object's lock.
In this example we may eliminate the deadlock by using two locks, one for short operations and one for long ones.
As already said here synchronized block can use user-defined variable as lock object, when synchronized function uses only "this". And of course you can manipulate with areas of your function which should be synchronized and so on.
But everyone says that no difference between synchronized function and block which covers whole function using "this" as lock object. That is not true, difference is in byte code which will be generated in both situations. In case of synchronized block usage should be allocated local variable which holds reference to "this". And as result we will have a little bit larger size of function (not relevant if you have only few number of functions).
More detailed explanation of the difference you can find here:
http://www.artima.com/insidejvm/ed2/threadsynchP.html
Also usage of synchronized block is not good due to following point of view:
The synchronized keyword is very limited in one area: when exiting a synchronized block, all threads that are waiting for that lock must be unblocked, but only one of those threads gets to take the lock; all the others see that the lock is taken and go back to the blocked state. That's not just a lot of wasted processing cycles: often the context switch to unblock a thread also involves paging memory off the disk, and that's very, very, expensive.
For more details in this area I would recommend you read this article:
http://java.dzone.com/articles/synchronized-considered
This is really just supplementary to the other answers, but if your main objection to using private objects for locking is that it clutters your class with fields that are not related to the business logic then Project Lombok has #Synchronized to generate the boilerplate at compile-time:
#Synchronized
public int foo() {
return 0;
}
compiles to
private final Object $lock = new Object[0];
public int foo() {
synchronized($lock) {
return 0;
}
}
A good example for use synchronized(this).
// add listener
public final synchronized void addListener(IListener l) {listeners.add(l);}
// remove listener
public final synchronized void removeListener(IListener l) {listeners.remove(l);}
// routine that raise events
public void run() {
// some code here...
Set ls;
synchronized(this) {
ls = listeners.clone();
}
for (IListener l : ls) { l.processEvent(event); }
// some code here...
}
As you can see here, we use synchronize on this to easy cooperate of lengthly (possibly infinite loop of run method) with some synchronized methods there.
Of course it can be very easily rewritten with using synchronized on private field. But sometimes, when we already have some design with synchronized methods (i.e. legacy class, we derive from, synchronized(this) can be the only solution).
It depends on the task you want to do, but I wouldn't use it. Also, check if the thread-save-ness you want to accompish couldn't be done by synchronize(this) in the first place? There are also some nice locks in the API that might help you :)
I only want to mention a possible solution for unique private references in atomic parts of code without dependencies. You can use a static Hashmap with locks and a simple static method named atomic() that creates required references automatically using stack information (full class name and line number). Then you can use this method in synchronize statements without writing new lock object.
// Synchronization objects (locks)
private static HashMap<String, Object> locks = new HashMap<String, Object>();
// Simple method
private static Object atomic() {
StackTraceElement [] stack = Thread.currentThread().getStackTrace(); // get execution point
StackTraceElement exepoint = stack[2];
// creates unique key from class name and line number using execution point
String key = String.format("%s#%d", exepoint.getClassName(), exepoint.getLineNumber());
Object lock = locks.get(key); // use old or create new lock
if (lock == null) {
lock = new Object();
locks.put(key, lock);
}
return lock; // return reference to lock
}
// Synchronized code
void dosomething1() {
// start commands
synchronized (atomic()) {
// atomic commands 1
...
}
// other command
}
// Synchronized code
void dosomething2() {
// start commands
synchronized (atomic()) {
// atomic commands 2
...
}
// other command
}
Avoid using synchronized(this) as a locking mechanism: This locks the whole class instance and can cause deadlocks. In such cases, refactor the code to lock only a specific method or variable, that way whole class doesn't get locked. Synchronised can be used inside method level.
Instead of using synchronized(this), below code shows how you could just lock a method.
public void foo() {
if(operation = null) {
synchronized(foo) {
if (operation == null) {
// enter your code that this method has to handle...
}
}
}
}
My two cents in 2019 even though this question could have been settled already.
Locking on 'this' is not bad if you know what you are doing but behind the scene locking on 'this' is (which unfortunately what synchronized keyword in method definition allows).
If you actually want users of your class to be able to 'steal' your lock (i.e. prevent other threads from dealing with it), you actually want all the synchronized methods to wait while another sync method is running and so on.
It should be intentional and well thought off (and hence documented to help your users understand it).
To further elaborate, in the reverse you must know what you are 'gaining' (or 'losing' out on) if you lock on a non accessible lock (nobody can 'steal' your lock, you are in total control and so on...).
The problem for me is that synchronized keyword in the method definition signature makes it just too easy for programmers not to think about what to lock on which is a mighty important thing to think about if you don't want to run into problems in a multi-threaded program.
One can't argue that 'typically' you don't want users of your class to be able to do these stuff or that 'typically' you want...It depends on what functionality you are coding. You can't make a thumb rule as you can't predict all the use cases.
Consider for e.g. the printwriter which uses an internal lock but then people struggle to use it from multiple threads if they don't want their output to interleave.
Should your lock be accessible outside of the class or not is your decision as a programmer on the basis of what functionality the class has. It is part of the api. You can't move away for instance from synchronized(this) to synchronized(provateObjet) without risking breaking changes in the code using it.
Note 1: I know you can achieve whatever synchronized(this) 'achieves' by using a explicit lock object and exposing it but I think it is unnecessary if your behaviour is well documented and you actually know what locking on 'this' means.
Note 2: I don't concur with the argument that if some code is accidentally stealing your lock its a bug and you have to solve it. This in a way is same argument as saying I can make all my methods public even if they are not meant to be public. If someone is 'accidentally' calling my intended to be private method its a bug. Why enable this accident in the first place!!! If ability to steal your lock is a problem for your class don't allow it. As simple as that.
Let me put the conclusion first - locking on private fields does not work for slightly more complicated multi-threaded program. This is because multi-threading is a global problem. It is impossible to localize synchronization unless you write in a very defensive way (e.g. copy everything on passing to other threads).
Here is the long explanation:
Synchronization includes 3 parts: Atomicity, Visibility and Ordering
Synchronized block is very coarse level of synchronization. It enforces visibility and ordering just as what you expected. But for atomicity, it does not provide much protection. Atomicity requires global knowledge of the program rather than local knowledge. (And that makes multi-threading programming very hard)
Let's say we have a class Account having method deposit and withdraw. They are both synchronized based on a private lock like this:
class Account {
private Object lock = new Object();
void withdraw(int amount) {
synchronized(lock) {
// ...
}
}
void deposit(int amount) {
synchronized(lock) {
// ...
}
}
}
Considering we need to implement a higher-level class which handles transfer, like this:
class AccountManager {
void transfer(Account fromAcc, Account toAcc, int amount) {
if (fromAcc.getBalance() > amount) {
fromAcc.setBalance(fromAcc.getBalance() - amount);
toAcc.setBalance(toAcc.getBalance + amount);
}
}
}
Assuming we have 2 accounts now,
Account john;
Account marry;
If the Account.deposit() and Account.withdraw() are locked with internal lock only. That will cause problem when we have 2 threads working:
// Some thread
void threadA() {
john.withdraw(500);
}
// Another thread
void threadB() {
accountManager.transfer(john, marry, 100);
}
Because it is possible for both threadA and threadB run at the same time. And thread B finishes the conditional check, thread A withdraws, and thread B withdraws again. This means we can withdraw $100 from John even if his account has no enough money. This will break atomicity.
You may propose that: why not adding withdraw() and deposit() to AccountManager then? But under this proposal, we need to create a multi-thread safe Map which maps from different accounts to their locks. We need to delete the lock after execution (otherwise will leak memory). And we also need to ensure no other one accesses the Account.withdraw() directly. This will introduce a lots of subtle bugs.
The correct and most idiomatic way is to expose the lock in the Account. And let the AccountManager to use the lock. But in this case, why not just use the object itself then?
class Account {
synchronized void withdraw(int amount) {
// ...
}
synchronized void deposit(int amount) {
// ...
}
}
class AccountManager {
void transfer(Account fromAcc, Account toAcc, int amount) {
// Ensure locking order to prevent deadlock
Account firstLock = fromAcc.hashCode() < toAcc.hashCode() ? fromAcc : toAcc;
Account secondLock = fromAcc.hashCode() < toAcc.hashCode() ? toAcc : fromAcc;
synchronized(firstLock) {
synchronized(secondLock) {
if (fromAcc.getBalance() > amount) {
fromAcc.setBalance(fromAcc.getBalance() - amount);
toAcc.setBalance(toAcc.getBalance + amount);
}
}
}
}
}
To conclude in simple English, private lock does not work for slightly more complicated multi-threaded program.
(Reposted from https://stackoverflow.com/a/67877650/474197)
I think points one (somebody else using your lock) and two (all methods using the same lock needlessly) can happen in any fairly large application. Especially when there's no good communication between developers.
It's not cast in stone, it's mostly an issue of good practice and preventing errors.