From what time I've spent with threads in Java, I've found these two ways to write threads:
With implements Runnable:
public class MyRunnable implements Runnable {
public void run() {
//Code
}
}
//Started with a "new Thread(new MyRunnable()).start()" call
Or, with extends Thread:
public class MyThread extends Thread {
public MyThread() {
super("MyThread");
}
public void run() {
//Code
}
}
//Started with a "new MyThread().start()" call
Is there any significant difference in these two blocks of code?
Yes: implements Runnable is the preferred way to do it, IMO. You're not really specialising the thread's behaviour. You're just giving it something to run. That means composition is the philosophically "purer" way to go.
In practical terms, it means you can implement Runnable and extend from another class as well... and you can also implement Runnable via a lambda expression as of Java 8.
tl;dr: implements Runnable is better. However, the caveat is important.
In general, I would recommend using something like Runnable rather than Thread because it allows you to keep your work only loosely coupled with your choice of concurrency. For example, if you use a Runnable and decide later on that this doesn't in fact require its own Thread, you can just call threadA.run().
Caveat: Around here, I strongly discourage the use of raw Threads. I much prefer the use of Callables and FutureTasks (From the javadoc: "A cancellable asynchronous computation"). The integration of timeouts, proper cancelling and the thread pooling of the modern concurrency support are all much more useful to me than piles of raw Threads.
Follow-up: There is a FutureTask constructor that allows you to use Runnables (if that's what you are most comfortable with) and still get the benefit of the modern concurrency tools. To quote the javadoc:
If you don't need a particular result, consider using constructions of the form:
Future<?> f = new FutureTask<Object>(runnable, null)
So, if we replace their runnable with your threadA, we get the following:
new FutureTask<Object>(threadA, null)
Another option that allows you to stay closer to Runnables is a ThreadPoolExecutor. You can use the execute method to pass in a Runnable to execute "the given task sometime in the future".
If you'd like to try using a thread pool, the code fragment above would become something like the following (using the Executors.newCachedThreadPool() factory method):
ExecutorService es = Executors.newCachedThreadPool();
es.execute(new ThreadA());
Moral of the story:
Inherit only if you want to override some behavior.
Or rather it should be read as:
Inherit less, interface more.
One thing that I'm surprised hasn't been mentioned yet is that implementing Runnable makes your class more flexible.
If you extend thread then the action you're doing is always going to be in a thread. However, if you implement Runnable it doesn't have to be. You can run it in a thread, or pass it to some kind of executor service, or just pass it around as a task within a single threaded application (maybe to be run at a later time, but within the same thread). The options are a lot more open if you just use Runnable than if you bind yourself to Thread.
If you want to implements or extends any other class then Runnable interface is most preferable, otherwise, if you do not want any other class to extend or implement then Thread class is preferable.
The most common difference is
When you extends Thread class, after that you can’t extend any other class which you required. (As you know, Java does not allow inheriting more than one class).
When you implements Runnable, you can save space for your class to extend any other class in the future or now.
Java doesn't support multiple inheritances, which means you can only extend one class in Java so once you extended Thread class you lost your chance and cannot extend or inherit another class in Java.
In Object-oriented programming, extending a class generally means, adding new functionality, and modifying or improving behaviors. If we are not making any modification on Thread then use Runnable interface instead.
Runnable interface represents a Task which can be executed by either plain Thread or Executors or any other means. so logical separation of Task as Runnable than Thread is a good design decision.
Separating task as Runnable means we can reuse the task and also has the liberty to execute it from different means. since you can not restart a Thread once it completes. again Runnable vs Thread for task, Runnable is winner.
Java designer recognizes this and that's why Executors accept Runnable as Task and they have worker thread which executes those task.
Inheriting all Thread methods are additional overhead just for representing a Task which can be done easily with Runnable.
Courtesy from javarevisited.blogspot.com
These were some of the notable differences between Thread and Runnable in Java. If you know any other differences on Thread vs Runnable than please share it via comments. I personally use Runnable over Thread for this scenario and recommends to use Runnable or Callable interface based on your requirement.
However, the significant difference is.
When you extends Thread class, each of your thread creates a unique object and associate with it.
When you implements Runnable, it shares the same object to multiple threads.
Actually, It is not wise to compare Runnable and Thread with each other.
This two have a dependency and relationship in multi-threading just like Wheel and Engine relationship of motor vehicle.
I would say, there is only one way for multi-threading with two steps. Let me make my point.
Runnable:
When implementing interface Runnable it means you are creating something which is run able in a different thread. Now creating something which can run inside a thread (runnable inside a thread), doesn't mean to creating a Thread.
So the class MyRunnable is nothing but a ordinary class with a void run method.
And it's objects will be some ordinary objects with only a method run which will execute normally when called. (unless we pass the object in a thread).
Thread:
class Thread, I would say A very special class with the capability of starting a new Thread which actually enables multi-threading through its start() method.
Why not wise to compare?
Because we need both of them for multi-threading.
For Multi-threading we need two things:
Something that can run inside a Thread (Runnable).
Something That can start a new Thread (Thread).
So technically and theoretically both of them is necessary to start a thread, one will run and one will make it run (Like Wheel and Engine of motor vehicle).
That's why you can not start a thread with MyRunnable you need to pass it to a instance of Thread.
But it is possible to create and run a thread only using class Thread because Class Thread implements Runnable so we all know Thread also is a Runnable inside.
Finally Thread and Runnable are complement to each other for multithreading not competitor or replacement.
You should implement Runnable, but if you are running on Java 5 or higher, you should not start it with new Thread but use an ExecutorService instead. For details see: How to implement simple threading in Java.
I'm not an expert, but I can think of one reason to implement Runnable instead of extend Thread: Java only supports single inheritance, so you can only extend one class.
Edit: This originally said "Implementing an interface requires less resources." as well, but you need to create a new Thread instance either way, so this was wrong.
I would say there is a third way:
public class Something {
public void justAnotherMethod() { ... }
}
new Thread(new Runnable() {
public void run() {
instanceOfSomething.justAnotherMethod();
}
}).start();
Maybe this is influenced a bit by my recent heavy usage of Javascript and Actionscript 3, but this way your class doesn't need to implement a pretty vague interface like Runnable.
With the release of Java 8, there is now a third option.
Runnable is a functional interface, which means that instances of it can be created with lambda expressions or method references.
Your example can be replaced with:
new Thread(() -> { /* Code here */ }).start()
or if you want to use an ExecutorService and a method reference:
executor.execute(runner::run)
These are not only much shorter than your examples, but also come with many of the advantages stated in other answers of using Runnable over Thread, such as single responsibility and using composition because you're not specializing the thread's behaviour. This way also avoids creating an extra class if all you need is a Runnable as you do in your examples.
Instantiating an interface gives a cleaner separation between your code and the implementation of threads, so I'd prefer to implement Runnable in this case.
Difference between Extending Thread and Implementing Runnable are:
Everyone here seems to think that implementing Runnable is the way to go and I don't really disagree with them but there is also a case for extending Thread in my opinion, in fact you have sort of demonstrated it in your code.
If you implement Runnable then the class that implements Runnable has no control over the thread name, it is the calling code that can set the thread name, like so:
new Thread(myRunnable,"WhateverNameiFeelLike");
but if you extend Thread then you get to manage this within the class itself (just like in your example you name the thread 'ThreadB'). In this case you:
A) might give it a more useful name for debugging purposes
B) are forcing that that name be used for all instances of that class (unless you ignore the fact that it is a thread and do the above with it as if it is a Runnable but we are talking about convention here in any case so can ignore that possibility I feel).
You might even for example take a stack trace of its creation and use that as the thread name. This might seem odd but depending on how your code is structured it can be very useful for debugging purposes.
This might seem like a small thing but where you have a very complex application with a lot of threads and all of a sudden things 'have stopped' (either for reasons of deadlock or possibly because of a flaw in a network protocol which would be less obvious - or other endless reasons) then getting a stack dump from Java where all the threads are called 'Thread-1','Thread-2','Thread-3' is not always very useful (it depends on how your threads are structured and whether you can usefully tell which is which just by their stack trace - not always possible if you are using groups of multiple threads all running the same code).
Having said that you could of course also do the above in a generic way by creating an extension of the thread class which sets its name to a stack trace of its creation call and then use that with your Runnable implementations instead of the standard java Thread class (see below) but in addition to the stack trace there might be more context specific information that would be useful in the thread name for debugging (a reference to one of many queues or sockets it could processing for example in which case you might prefer to extend Thread specifically for that case so that you can have the compiler force you (or others using your libraries) to pass in certain info (e.g. the queue/socket in question) for use in the name).
Here's an example of the generic thread with the calling stack trace as its name:
public class DebuggableThread extends Thread {
private static String getStackTrace(String name) {
Throwable t= new Throwable("DebuggableThread-"+name);
ByteArrayOutputStream os = new ByteArrayOutputStream();
PrintStream ps = new PrintStream(os);
t.printStackTrace(ps);
return os.toString();
}
public DebuggableThread(String name) {
super(getStackTrace(name));
}
public static void main(String[] args) throws Exception {
System.out.println(new Thread());
System.out.println(new DebuggableThread("MainTest"));
}
}
and here's a sample of the output comparing the two names:
Thread[Thread-1,5,main]
Thread[java.lang.Throwable: DebuggableThread-MainTest
at DebuggableThread.getStackTrace(DebuggableThread.java:6)
at DebuggableThread.<init>(DebuggableThread.java:14)
at DebuggableThread.main(DebuggableThread.java:19)
,5,main]
Runnable because:
Leaves more flexibility for the
Runnable implementation to extend
another class
Separates the code from
execution
Allows you to run your
runnable from a Thread Pool, the
event thread, or in any other way in
the future.
Even if you don't need any of this now, you may in the future. Since there is no benefit to overriding Thread, Runnable is a better solution.
Since this is a very popular topic and the good answers are spread all over and dealt with in great depth, I felt it is justifiable to compile the good answers from the others into a more concise form, so newcomers have an easy overview upfront:
You usually extend a class to add or modify functionality. So, if you don't want to overwrite any Thread behavior, then use Runnable.
In the same light, if you don't need to inherit thread methods, you can do without that overhead by using Runnable.
Single inheritance: If you extend Thread you cannot extend from any other class, so if that is what you need to do, you have to use Runnable.
It is good design to separate domain logic from technical means, in that sense it is better to have a Runnable task isolating your task from your runner.
You can execute the same Runnable object multiple times, a Thread object, however, can only be started once. (Maybe the reason, why Executors do accept Runnables, but not Threads.)
If you develop your task as Runnable, you have all flexibility how to use it now and in the future. You can have it run concurrently via Executors but also via Thread. And you still could also use/call it non-concurrently within the same thread just as any other ordinary type/object.
This makes it also easier to separate task-logic and concurrency aspects in your unit tests.
If you are interested in this question, you might be also interested in the difference between Callable and Runnable.
This is discussed in Oracle's Defining and Starting a Thread tutorial:
Which of these idioms should you use? The first idiom, which employs a
Runnable object, is more general, because the Runnable object can
subclass a class other than Thread. The second idiom is easier to use
in simple applications, but is limited by the fact that your task
class must be a descendant of Thread. This lesson focuses on the first
approach, which separates the Runnable task from the Thread object
that executes the task. Not only is this approach more flexible, but
it is applicable to the high-level thread management APIs covered
later.
In other words, implementing Runnable will work in scenarios where your class extends a class other than Thread. Java does not support multiple inheritance. Also, extending Thread will not be possible when using some of the high-level thread management APIs. The only scenario where extending Thread is preferable is in a small application that won't be subject to updates in future. It is almost always better to implement Runnable as it is more flexible as your project grows. A design change won't have a major impact as you can implement many interfaces in java, but only extend one class.
The simplest explanation would be by implementing Runnable we can assign the same object to multiple threads and each Thread shares the same object states and behavior.
For example, suppose there are two threads, thread1 puts an integer in an array and thread2 takes integers from the array when the array is filled up. Notice that in order for thread2 to work it needs to know the state of array, whether thread1 has filled it up or not.
Implementing Runnable lets you to have this flexibility to share the object whereas extends Thread makes you to create new objects for each threads therefore any update that is done by thread1 is lost to thread2.
If I am not wrong, it's more or less similar to
What is the difference between an interface and abstract class?
extends establishes "Is A" relation & interface provides "Has a" capability.
Prefer implements Runnable :
If you don't have to extend Thread class and modify Thread API default implementation
If you are executing a fire and forget command
If You are already extending another class
Prefer "extends Thread" :
If you have to override any of these Thread methods as listed in oracle documentation page
Generally you don't need to override Thread behaviour. So implements Runnable is preferred for most of the times.
On a different note, using advanced ExecutorService or ThreadPoolExecutorService API provides more flexibility and control.
Have a look at this SE Question:
ExecutorService vs Casual Thread Spawner
Separating the Thread class from the Runnable implementation also avoids potential synchronization problems between the thread and the run() method. A separate Runnable generally gives greater flexibility in the way that runnable code is referenced and executed.
Runnable is an interface, while Thread is a class which implements this interface. From a design point of view, there should be a clean separation between how a task is defined and between how it is executed. The former is the responsibility of a Runnalbe implementation, and the latter is job of the Thread class. In most of the cases implementing Runnable is the right way to follow.
That's the S of SOLID: Single responsibility.
A thread embodies the running context (as in execution context: stack frame, thread id, etc.) of the asynchronous execution of a piece of code. That piece of code ideally should be the same implementation, whether synchronous or asynchronous.
If you bundle them together in one implementation, you give the resulting object two unrelated causes of change:
thread handling in your application (ie. querying and modifying the execution context)
algorithm implemented by the piece of code (the runnable part)
If the language you use supports partial classes or multiple inheritance, then you can segregate each cause in its own super class, but it boils down to the same as composing the two objects, since their feature sets don't overlap. That's for the theory.
In practice, generally speaking, a programme does not need to carry more complexity than necessary. If you have one thread working on a specific task, without ever changing that task, there is probably no point in making the tasks separate classes, and your code remains simpler.
In the context of Java, since the facility is already there, it is probably easier to start directly with stand alone Runnable classes, and pass their instances to Thread (or Executor) instances. Once used to that pattern, it is not harder to use (or even read) than the simple runnable thread case.
One reason you'd want to implement an interface rather than extend a base class is that you are already extending some other class. You can only extend one class, but you can implement any number of interfaces.
If you extend Thread, you're basically preventing your logic to be executed by any other thread than 'this'. If you only want some thread to execute your logic, it's better to just implement Runnable.
if you use runnable you can save the space to extend to any of your other class.
Can we re-visit the basic reason we wanted our class to behave as a Thread?
There is no reason at all, we just wanted to execute a task, most likely in an asynchronous mode, which precisely means that the execution of the task must branch from our main thread and the main thread if finishes early, may or may not wait for the branched path(task).
If this is the whole purpose, then where do I see the need of a specialized Thread. This can be accomplished by picking up a RAW Thread from the System's Thread Pool and assigning it our task (may be an instance of our class) and that is it.
So let us obey the OOPs concept and write a class of the type we need. There are many ways to do things, doing it in the right way matters.
We need a task, so write a task definition which can be run on a Thread. So use Runnable.
Always remember implements is specially used to impart a behaviour and extends is used to impart a feature/property.
We do not want the thread's property, instead we want our class to behave as a task which can be run.
Yes,
If you call ThreadA call , then not need to call the start method and run method is call after call the ThreadA class only.
But If use the ThreadB call then need to necessary the start thread for call run method.
If you have any more help, reply me.
I find it is most useful to use Runnable for all the reasons mentioned, but sometimes I like to extend Thread so I can create my own thread stopping method and call it directly on the thread I have created.
Java does not support multiple inheritence so if you extends Thread class then no other class will be extended.
For Example: If you create an applet then it must extends Applet class so here the only way to create thread is by implementing Runnable interface
Difference between Thread and runnable
.If we are creating Thread using Thread class then Number of thread equal to number of object we created .
If we are creating thread by implementing the runnable interface then we can use single object for creating multiple thread.So single object is shared by multiple Thread.So it will take less memory
So depending upon the requirement if our data is not senstive. So It can be shared between multiple Thread we can used Runnable interface.
Adding my two cents here -
Always whenever possible use implements Runnable . Below are two caveats on why you should not use
extends Threads
Ideally you should never extend the Thread class; the Thread class should be made final.
At least its methods like thread.getId().
See this discussion for a bug related to extending Threads.
Those who like to solve puzzles can see another side effect of extending Thread. The below code
will print unreachable code when nobody is notifying them.
Please see http://pastebin.com/BjKNNs2G.
public class WaitPuzzle {
public static void main(String[] args) throws InterruptedException {
DoNothing doNothing = new DoNothing();
new WaitForever(doNothing).start();
new WaitForever(doNothing).start();
new WaitForever(doNothing).start();
Thread.sleep(100);
doNothing.start();
while(true) {
Thread.sleep(10);
}
}
static class WaitForever extends Thread {
private DoNothing doNothing;
public WaitForever(DoNothing doNothing) {
this.doNothing = doNothing;
}
#Override
public void run() {
synchronized (doNothing) {
try {
doNothing.wait(); // will wait forever here as nobody notifies here
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Unreachable Code");
}
}
}
static class DoNothing extends Thread {
#Override
public void run() {
System.out.println("Do Nothing ");
}
}
}
One difference between implementing Runnable and extending Thread is that by extending Thread, each of your threads has a unique object associated with it, whereas implementing Runnable, many threads can share the same object instance.
A class that implements Runnable is not a thread and just a class. For a Runnable to be executed by a Thread, you need to create an instance of Thread and pass the Runnable instance in as the target.
In most cases, the Runnable interface should be used if you are only planning to override the run() method and no other Thread methods. This is important because classes should not be subclassed unless the programmer intends on modifying or enhancing the fundamental behavior of the class.
When there is a need to extend a superclass, implementing the Runnable interface is more appropriate than using the Thread class. Because we can extend another class while implementing Runnable interface to make a thread. But if we just extend the Thread class we can't inherit from any other class.
Is it possible in Java (Android) to implement a customized version of a Thread which carries its own States?
What I mean is:
While ThreadA is in Running state, it still can be polled by ThreadB that asks for its state
e.g.
ThreadA.getState();
It is possible to modify the states values to some custom ones? So as to implement a sort of basic communication system between those two threads?
Thanks.
Yes that is possible. I used this a lot in my previous projects, all what you need is to extend the Thread class.
public class StateThread extends Thread{
String state = "ThreadState";
public synchronized void setState(String newState){
state = newState;
}
public synchronized String getState(){
return state;
}
#override
public void run(){
// Do stuff and update state...
}
}
Yes, it is possible to perform this task.
Is it a good design? I don't think so.
There are other means to perform communication between threads -
For example, you should use a queue with a Producer/Consumer pattern.
I am sure that Android, as JavaSE supports thread local - you can use it in order to manage local thread data (including states) (maybe in combination with a queue that will get "operations" to change the state managed by a thread
If you do decide to go for the solution of having setState and getState methods, at least consider using the ReaderWriterLock to optimize your locking
Threads state is maintained by the Virtual Machine. VM uses the state to monitor and manage the actual thread.
That's why there is no mechanism to modify the state of the Thread. There is no setState function that allows to set your custom state.
For your application purpose, you can define your own instance variables by extending Thread but that cannot alter Thread's state in any way.
Synchronizing with shared data is not very useful for determining the 'state' of a thread - the thread writes its state as 'healthy', then gets stuck - the monitor thread then checks the state and finds it healthy.
Monitoring the 'state' should mean making the checked thread do something, not just looking directly at some shared object.
If you have a message-passing design, (as suggested by zaske), you can pass around a 'state record' on the input queue of evey thread, asking it to record its state inside and pass it on to the next thread. The 'monitor' thread waits for the record to come back, all filled in. If it does not get it in a resonable time, it could log what it has got - it keeps a reference to the state record object, so it could see which thread has not updated its state. It could, perhaps, fail to feed a watchdog timer.
I am not understanding this concept in any manner.
public class SomeName {
public static void main(String args[]) {
}
}
This is my class SomeName. Now what is thread here.
Do we call the class as a thread.
Do we call this class as thread when some other object is trying to access its method or members?
Do we call this class as thread when some other object is trying to access this object?
What does it mean when we call something in java as thread-safe ?
Being thread-safe means avoiding several problems. The most common and probably the worst is called threadlock. The old analogy is the story of the dining philosophers. They are very polite and will never reach out their chopsticks to take food when someone else is doing the same. If they all reach out at the same time, then they all stop at the same time, and wait...and nothing ever happens, because they're all too polite to go first.
As someone else pointed out, if your app never creates additional threads, but merely runs from a main method, then there is only one thread, or one "dining philosopher," so threadlock can't occur. When you have multiple threads, the simplest way to avoid threadlock is to use a "monitor", which is just an object that's set aside. In effect, your methods have to obtain a "lock" on this monitor before accessing threads, so there are no collisions. However, you can still have threadlock, because there might be two objects trying to access two different threads, each with its own monitor. Object A has to wait for Object B to release its lock on monitor object 1; Object B has to wait for Object A to release its lock on monitor object 2. So now you're back to threadlock.
In short, thread safety is not terribly difficult to understand, but it does take time, practice and experience. The first time you write a multi-threaded app, you will run into threadlock. Then you will learn, and it soon becomes pretty intuitive. The biggest caveat is that you need to keep the multi-threaded parts of an app as simple as possible. If you have lots of threads, with lots of monitors and locks, it becomes exponentially more difficult to ensure that your dining philosophers never freeze.
The Java tutorial goes over threading extremely well; it was the only resource I ever needed.
You might want to think of thread as CPU executing the code that you wrote.
What is thread?
A thread is a single sequential flow of control within a program.
From Java concurrency in practice:
Thread-safe classes encapsulate any needed synchronization so that
clients need not provide their own.
At any time you have "execution points" where the JVM is running your code stepping through methods and doing what your program tells it to do.
For simple programs you only have one. For more complex programs you can have several, usually invoked with a new Thread().run or an Executor.
"Thread-safe" refers to that your code is written in such a way that one execution point cannot change what another execution point sees. This is usually very desirable as these changes can be very hard to debug, but as you only have one, there is not another so this does not apply.
Threads is an advanced subject which you will come back to later, but for now just think that if you do not do anything special with Threads or Swing this will not apply to you. It will later, but not now.
Well, in your specific example, when your program runs, it has just 1 thread.
The main thread.
A class is thread safe when an object of that class can be accessed in parallel from multiple threads (and hence from multiple CPUs) without any of the guarantees that it would provide in a single threaded way to be broken.
You should read first about what exactly threads are, for instance on Wikipedia, which might make it then easier to understand the relation between classes and threads and the notion of threadsafety.
Every piece of code in Java is executed on some thread. By default, there is a "main" thread that calls your main method. All code in your program executes on the main thread unless you create another thread and start it. Threads start when you explicitly call the Thread.start() method; they can also start implicitly when you call an API that indirectly calls Thread.start(). (API calls that start a thread are generally documented to do so.) When Thread.start() is called, it creates a new thread of execution and calls the Thread object's run() method. The thread exits when its run() method returns.
There are other ways to affect threads, but that's the basics. You can read more details in the Java concurrency tutorial.
In putting together a simple "Clock" application I discovered that Android requires you to use android.os.Handler that lives in Thread A in order to update View objects in Thread A with the results of data that come from Thread B.
I'm still relatively new to Java, and this is the first time I've dealt explicitly with Threads before, but I know that ordinarily you'd declare methods and/or operations as synchronized if two different threads want to access the same data. It appears though, that android.os.Handler is an Android-specific way of synchronizing data between threads such that you avoid the classic concurrency errors detailed in the Oracle documentation I just linked to. Is this really true?
If I were to find myself in a situation such that I were required by the Android OS to use android.os.Handler to ferry data from one thread to another, does that mean I don't have to declare the methods used to derive that data as synchronized?
My understanding:
A Handler is just a mechanism for delivering information between two threads. It's not the only mechanism, but it is the mechanism Google chose to use for adding easy to use methods to Activities to do common tasks.
From the doc
When a process is created for your
application, its main thread is
dedicated to running a message queue
that takes care of managing the
top-level application objects
(activities, broadcast receivers, etc)
and any windows they create. You can
create your own threads, and
communicate back with the main
application thread through a Handler.
This is done by calling the same post
or sendMessage methods as before, but
from your new thread. The given
Runnable or Message will then be
scheduled in the Handler's message
queue and processed when appropriate.
The main thread is running a Handler. The handler is responsible for running your activities and managing the application environment. The Handler is just a message loop which dispatches things from a MessageQueue. This is why your thread has to be running a Looper to create a Handler. To synchronize other stuff you want to do with this effort, you have to insert your requests into this message queue so the main application thread does your work. The ui is written so as long as only one thread accesses the objects, it functions.
I guess the point of this was to point out that the Handler is one mechanism for doing synchronization. Surely, the internals of the MessageQueue are written to take synchronization into account, though its a gross simplification to say 'if multiple threads access it it needs to be synchronized.' Although the UI is the most frequent example, using a Handler and Looper is simply a mechanism for designing a multi-threaded application which provides single thread synchronization for the processing of events.
Whether or not you need to synchronize what you send to a Handler depends greatly on what you are sending. If you are talking about synchronizing the calls to post stuff, then no, the handler takes care of that. If you're talking about code inside a Runnable that you post to a Handler, realize that the only guarantee of the Handler is that your runnable will be executed using the thread that created the Handler. You will need to continue to provide synchronization with other resources.
You only ever need synchronized if you have a shared resource, like an ArrayList or something that can be read from two threads at the same time.
The Handler by itself doesn't prevent any concurrency, it just makes it a lot easier to perform things that need to happen in the UI thread even though a worker thread is trying to do them.
To answer your question: If you use a Handler, it typically means that you do certain critical things in the UI thread. As an example, you have an ArrayList that you initialize in onCreate and then maybe update in a click handler or something. Now if you use a Handler to modify that, then ALL access to the ArrayList will occur in the UI thread, so there is no need for synchronized.
However, as soon as you access that ArrayList from within a worker thread, you WILL need to synchronize every single access to it.
Synchronized, wait and notify are low level concurrency constructs on top of which high level concurrency constructs like semaphores, blocking queues, barriers and Looper/Handler are implemented.
Most of what you see in java.util.concurrent (which implements high level concurrency patterns/constructs) is implemented on top of synchronized, wait and notify (if you ignore the lock free algorithms)
Looper/Handler mechanism in basically a lightweight implementation of producer-consumer pattern where you have one consumer but multiple producers. It was created to easily exchange messages between the UI thread and non UI threads. UI thread in android runs as a single threaded loop (i.e single consumer) but can take in updates from multiple non UI threads (i.e multiple producers via handlers).