class A{
synchronized static void method(){
doSomethingLongTime(); // here A.class monitior is taken.
}
}
.......
new A(); // does this blocked by doSomethingLongTime() ?
The code above depicts my question: new A() definetly deal with A.class, so is it blocked?
Nope. The lock on the static method is acquired on A class object. That is right.
But new A() is not inside synchronized block. So this line doesn't need to wait for any lock object & can proceed. Construction of new object won't be blocked otherwise explicitly specified within a synchronized block.
Related
Say we have a object foo:
class Foo(){
public synchronized void instanceMethod(){}
}
var foo = new Foo();
if I have a lock on foo:
synchronized(foo){
foo.instanceMethod();
}
do I also have a lock on the instanceMethod() call? Another way of asking the question - if I have a lock on foo, can another thread call foo.instanceMethod() (simultaneously)?
if I have a lock on foo, can another thread call foo.instanceMethod()?
They can call it, but the call will wait until execution leaves your block synchronized on foo, because instanceMethod is synchronized. Declaring an instance method synchronized is roughly the same as putting its entire body in a block synchronized on this.
If instanceMethod weren't synchronized, then of course the call wouldn't wait.
Note, though, that the synchronized block you've shown is unnecessary:
synchronized(foo){ // <==== Unnecessary
foo.instanceMethod();
}
Because instanceMethod is synchronized, that can just be:
foo.instanceMethod();
...unless there's something else in the block as well.
class Foo {
public synchronized void a() { //Do something }
public void b() {
synchronized(this) { // Do something }
}
public void c() { // Do something }
}
Then:
Foo foo = new Foo();
foo.a();
foo.b();
synchronized(foo) { foo.c(); }
All the 3 methods are all pretty much equivalent in terms of synchronization.
There is no such thing as "locking" a method. Locking is done only on objects. Marking a method synchronized simply makes it lock the instance (or its class object for static method).
When you access a method on a locked object, the execution would be blocked as the thread fails to retrieve the monitor for the specified object - that is even before the method has been called. So foo.a() would be blocked when it is getting the foo.
Adding on...
I suddenly remembered something. If you have thread A calling foo.a() and it is taking a very long time to complete, and at that time another thread calls foo.c(), then foo.c() will still be blocked until foo.a() completes.
I saw this in a tutorial, they asked if there is a problem with the following code. To me it looked like b() cannot be accessed as a() is already having the control over the monitor. Am I right in thinking so?
public class Test {
public synchronized void a() {
b();
System.out.println("I am at a");
}
public synchronized void b() {
System.out.println("I am at b");
}
}
No, there is no problem with that code.
Note two things:
synchronized SomeType foo() { ... } is equivalent to
SomeType foo() {
synchronized (this) { ... }
}
It locks the this instance of the enclosing class. So, in your case a() and b() are locking the same thing
If a thread is already holding a lock on some object's monitor, it prevents another thread from acquiring a lock on the same object, but the same thread can acquire more locks if it needs too, that is not affected. So
public synchronized void a() { // acquires lock on this
b(); // also aquires lock on this, but it's ok because it is the same thread
System.out.println("I am at a");
}
While a thread is inside a(), no other thread will be able to call either a() or b() on the same instance. If they try to, they will have to wait until the current thread exits a(). But the current thread itself is not affected, it can call any synchronized method on this object, because it is already holding the lock.
No, you're incorrect. The thread has control of the monitor, not the method, and so it's capable of following execution into as many methods synchronized on the same object as needed.
What is the difference between synchronizing a static method and a non static method in java?Can anybody please explain with an example. Also is there any difference in synchronizing a method and synchronizing a block of code?
I will try and add an example to make this extra clear.
As has been mentioned, synchronized in Java is an implementation of the Monitor concept. When you mark a block of code as synchronized you use an object as a parameter. When an executing thread comes to such a block of code, it has to first wait until there is no other executing thread in a synchronized block on that same object.
Object a = new Object();
Object b = new Object();
...
synchronized(a){
doStuff();
}
...
synchronized(b){
doSomeStuff();
}
...
synchronized(a){
doOtherStuff();
}
In the above example, a thread running doOtherStuff() would block another thread from entering the block of code protecting doStuff(). However, a thread could enter the block around doSomeStuff() without a problem as that is synchronized on Object b, not Object a.
When you use the synchronized modifier on an instance method (a non-static method), it is very similar to having a synchronized block with "this" as the argument. So in the following example, methodA() and methodB() will act the same way:
public synchronized void methodA() {
doStuff();
}
...
public void methodB() {
synchronized(this) {
doStuff();
}
}
Note that if you have a methodC() in that class which is not synchronized and does not have a synchronized block, nothing will stop a thread from entering that method and careless programming could let that thread access non-safe code in the object.
If you have a static method with the synchronized modifier, it is practically the same thing as having a synchronized block with ClassName.class as the argument (if you have an object of that class, ClassName cn = new ClassName();, you can access that object with Class c = cn.getClass();)
class ClassName {
public void static synchronized staticMethodA() {
doStaticStuff();
}
public static void staticMethodB() {
synchronized(ClassName.class) {
doStaticStuff();
}
}
public void nonStaticMethodC() {
synchronized(this.getClass()) {
doStuff();
}
}
public static void unSafeStaticMethodD() {
doStaticStuff();
}
}
So in the above example, staticMethodA() and staticMethodB() act the same way. An executing thread will also be blocked from accessing the code block in nonStaticMethodC() as it is synchronizing on the same object.
However, it is important to know that nothing will stop an executing thread from accessing unSafeStaticMethodD(). Even if we say that a static method "synchronizes on the Class object", it does not mean that it synchronizes all accesses to methods in that class. It simply means that it uses the Class object to synchronize on. Non-safe access is still possible.
In short if you synchronize on a static method you will synchronize on the class (object) and not on an instance (object). That means while execution of a static method the whole class is blocked. So other static synchronized methods are also blocked.
Synchronization in Java is basically an implementation of monitors. When synchronizing a non static method, the monitor belongs to the instance. When synchronizing on a static method, the monitor belongs to the class. Synchronizing a block of code is the same idea, but the monitor belongs to the specified object. If you can get away with it, synchronized blocks are preferable because they minimize the time each thread spends in the critical section
There is virtually no difference between synchronizing a block and synchronizing a method. Basically:
void synchronized m() {...}
is the same as
void m() { synchronized(this) {...} }
By comparison a static synchronized method is the same as:
static void m() { synchronized(MyClass.class) {...} }
Dude, just a hint. Not related to your question:
If any do*Stuff() methods does either
this.a= /*yet another*/ new Object();
or
this.b= /*yet another*/ new Object();
then you are screwed. Because the lock is inside the value, not inside the reference. See Java synchronized references
From javadoc https://docs.oracle.com/javase/tutorial/essential/concurrency/locksync.html
when a static synchronized method is invoked, since a static method is associated with a class, not an object. In this case, the thread acquires the intrinsic lock for the Class object associated with the class. Thus access to class's static fields is controlled by a lock that's distinct from the lock for any instance of the class.
public static synchronized void getInstance(){}
When we acquire a lock on any class, we actually acquire a lock on "Class" class instance which is only one for all instances of class.
public synchronized void getInstance(){}
we can create multiple object's of a class and each object will have one lock associated with it.
If I have the following code
class SomeClass {
...
public synchronized methodA() {
....
}
public synchronized methodB(){
....
}
}
This would synchronized on the 'this' object.
However, if my main objective here is to make sure multiple threads don't use methodA (or methodB) at the same time, but they CAN use methodA AND methodB concurrently,
then is this kind of design restrictive? since here thread1 lock the object (monitor object associated with the object) for running methodA but meanwhile thread2 is also waiting on the object lock even though methodA and methodB can run concurrently.
Is this understanding correct?
If yes, is this the kind of situation where we use synchronized block on a private dummy object so that methodA and methodB can run parallely with different thread but not methodA (or methodB) with different threads.
Thanks.
You've answered the question yourself: use one lock object per method and you're safe.
private final Object lockA = new Object();
private final Object lockB = new Object();
public void methodA() {
synchronized(lockA){
....
}
}
public void methodB() {
synchronized(lockB){
....
}
}
For more advanced locking mechanisms (e.g. ReentrantLock), read Java Concurrency in Practice by Brian Goetz et al. You should also read Effective Java by Josh Bloch, it also contains some items about using synchronized.
If you want to allow running methodA() and methodB() concurrently but otherwise restrict each method to one thread then you need two separate objects to synchronize on. For instance:
class SomeClass {
private final Object lockA = new Object();
private final Object lockB = new Object();
public void methodA() {
synchronized (lockA) {
//
}
}
public void methodB() {
synchronized (lockB) {
//
}
}
}
If my understanding is correct you want to allow thread T1 to run methodA() at the same time thread T2 runs methodB() -- but you don't want thread T1 to run methodA() at the same time thread T2 runs methodA() (and same for methodB) right?
For this scenario you can't use just a simple synchronized method -- instead, as you said, you will need 2 dummy objects (one for methodA and one for methodB) to synchronize on. Or you could use the new Lock class -- one Lock instance per method.
I want to understand how locking is done on static methods in Java.
let's say I have the following class:
class Foo {
private static int bar = 0;
public static synchronized void inc() { bar++; }
public synchronized int get() { return bar; }
It's my understanding that when I call f.get(), the thread acquires the lock on the object f and when I do Foo.inc() the thread acquires the lock on the class Foo.
My question is how are the two calls synchronized in respect to each other?
Is calling a static method also acquires a lock on all instantiations, or the other way around (which seems more reasonable)?
EDIT:
My question isn't exactly how static synchronized works, but how does static and non-static methods are synchronized with each other.
i.e., I don't want two threads to simultaneously call both f.get() and Foo.inc(), but these methods acquire different locks. My question is how is this preventable and is it prevented in the above code.
Static and instance synchronized methods are not related to each other, therefore you need to apply some additional synchronization between them, like this:
class Foo {
private static int bar = 0;
public static synchronized void inc() { bar++; }
public synchronized int get() {
synchronized (Foo.class) { // Synchronizes with static synchronized methods
return bar;
}
}
}
(though in this case leaving synchronized on get() doesn't make sense, since it doesn't do anything that requires synchronization on instance).
Beware of deadlocks - since this code aquires multiple locks, it should do it in consistent order, i.e. other synchronized static methods shouldn't try to acquire instance locks.
Also note that this particular task can be solved without synchronization at all, using atomic fields:
class Foo {
private static AtomicInteger bar = new AtomicInteger(0);
public static void inc() { bar.getAndIncrement(); }
public int get() { return bar.get(); }
}
A synchronized static method is effectively equivalent to:
public static void foo() {
synchronized (ClassName.class) {
// Body
}
}
In other words, it locks on the Class object associated with the class declaring the method.
From section 8.4.3.6 of the JLS:
A synchronized method acquires a monitor (ยง17.1) before it executes. For a class (static) method, the monitor associated with the Class object for the method's class is used. For an instance method, the monitor associated with this (the object for which the method was invoked) is used.
If you read http://download.oracle.com/javase/tutorial/essential/concurrency/locksync.html.
It will tell you:
You might wonder what happens when a
static synchronized method is invoked,
since a static method is associated
with a class, not an object. In this
case, the thread acquires the
intrinsic lock for the Class object
associated with the class. Thus access
to class's static fields is controlled
by a lock that's distinct from the
lock for any instance of the class.
which tells you all you need to know.
Neither, the non-static synchronized call does not acquire a lock on the class itself. (And the static synchronized block does not lock any object instantiated from that class.)
In other words the calls f.get() (locks f) and Foo.inc() (locks the class Foo) can run concurrently. They are not "synchronized".
You could use a different pattern (singleton), or make all the methods static.
Static locks are attached to the class definition and thus is shared between all instances of that class.
Synchronization of none static methods only apply to the current instance of the class (the lock is on the class instance, e.g., this). In your example you have two different locks with no interrelation.
I don't want two threads to simultaneously call both f.get() and Foo.inc(), but these methods acquire different locks. My question is how is this preventable and is it prevented in the above code
You must share a lock to be able to arbitrate access to both f.get and Foo.inc(). You can do this either by sharing the same static lock or by the same instance lock.
These two calls do not synchronize in respect to each other.
It is as you said, a caller of f.get() acquires the lock of f object and caller of Foo.inc() acquires Foo.class object's one. So the synchronization rules are the same as if instead of static call you called an instance synchronized method with another object.