[java] What does 'synchronized' mean?

I have some questions regarding the usage and significance of the synchronized keyword.

• What is the significance of the synchronized keyword?
• When should methods be synchronized?
• What does it mean programmatically and logically?

What the other answers are missing is one important aspect: memory barriers. Thread synchronization basically consists of two parts: serialization and visibility. I advise everyone to google for "jvm memory barrier", as it is a non-trivial and extremely important topic (if you modify shared data accessed by multiple threads). Having done that, I advise looking at java.util.concurrent package's classes that help to avoid using explicit synchronization, which in turn helps keeping programs simple and efficient, maybe even preventing deadlocks.

One such example is ConcurrentLinkedDeque. Together with the command pattern it allows to create highly efficient worker threads by stuffing the commands into the concurrent queue -- no explicit synchronization needed, no deadlocks possible, no explicit sleep() necessary, just poll the queue by calling take().

In short: "memory synchronization" happens implicitly when you start a thread, a thread ends, you read a volatile variable, you unlock a monitor (leave a synchronized block/function) etc. This "synchronization" affects (in a sense "flushes") all writes done before that particular action. In the case of the aforementioned ConcurrentLinkedDeque, the documentation "says":

Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a ConcurrentLinkedDeque happen-before actions subsequent to the access or removal of that element from the ConcurrentLinkedDeque in another thread.

This implicit behavior is a somewhat pernicious aspect because most Java programmers without much experience will just take a lot as given because of it. And then suddenly stumble over this thread after Java isn't doing what it is "supposed" to do in production where there is a different work load -- and it's pretty hard to test concurrency issues.

To my understanding synchronized basically means that the compiler write a monitor.enter and monitor.exit around your method. As such it may be thread safe depending on how it is used (what I mean is you can write an object with synchronized methods that isn't threadsafe depending on what your class does).

Synchronized simply means that multiple threads if associated with single object can prevent dirty read and write if synchronized block is used on particular object. To give you more clarity , lets take an example :

class MyRunnable implements Runnable {
    int var = 10;
    @Override
    public void run() {
        call();
    }

    public void call() {
        synchronized (this) {
            for (int i = 0; i < 4; i++) {
                var++;
                System.out.println("Current Thread " + Thread.currentThread().getName() + " var value "+var);
            }
        }
    }
}

public class MutlipleThreadsRunnable {
    public static void main(String[] args) {
        MyRunnable runnable1 = new MyRunnable();
        MyRunnable runnable2 = new MyRunnable();
        Thread t1 = new Thread(runnable1);
        t1.setName("Thread -1");
        Thread t2 = new Thread(runnable2);
        t2.setName("Thread -2");
        Thread t3 = new Thread(runnable1);
        t3.setName("Thread -3");
        t1.start();
        t2.start();
        t3.start();
    }
}

We've created two MyRunnable class objects , runnable1 being shared with thread 1 and thread 3 & runnable2 being shared with thread 2 only. Now when t1 and t3 starts without synchronized being used , PFB output which suggest that both threads 1 and 3 simultaneously affecting var value where for thread 2 , var has its own memory.

Without Synchronized keyword

    Current Thread Thread -1 var value 11
    Current Thread Thread -2 var value 11
    Current Thread Thread -2 var value 12
    Current Thread Thread -2 var value 13
    Current Thread Thread -2 var value 14
    Current Thread Thread -1 var value 12
    Current Thread Thread -3 var value 13
    Current Thread Thread -3 var value 15
    Current Thread Thread -1 var value 14
    Current Thread Thread -1 var value 17
    Current Thread Thread -3 var value 16
    Current Thread Thread -3 var value 18

Using Synchronzied, thread 3 waiting for thread 1 to complete in all scenarios. There are two locks acquired , one on runnable1 shared by thread 1 and thread 3 and another on runnable2 shared by thread 2 only.

Current Thread Thread -1 var value 11
Current Thread Thread -2 var value 11
Current Thread Thread -1 var value 12
Current Thread Thread -2 var value 12
Current Thread Thread -1 var value 13
Current Thread Thread -2 var value 13
Current Thread Thread -1 var value 14
Current Thread Thread -2 var value 14
Current Thread Thread -3 var value 15
Current Thread Thread -3 var value 16
Current Thread Thread -3 var value 17
Current Thread Thread -3 var value 18

synchronized means that in a multi threaded environment, an object having synchronized method(s)/block(s) does not let two threads to access the synchronized method(s)/block(s) of code at the same time. This means that one thread can't read while another thread updates it.

The second thread will instead wait until the first thread completes its execution. The overhead is speed, but the advantage is guaranteed consistency of data.

If your application is single threaded though, synchronized blocks does not provide benefits.

The synchronized keyword prevents concurrent access to a block of code or object by multiple threads. All the methods of Hashtable are synchronized, so only one thread can execute any of them at a time.

When using non-synchronized constructs like HashMap, you must build thread-safety features in your code to prevent consistency errors.

In java to prevent multiple threads manipulating a shared variable we use synchronized keyword. Lets understand it with help of the following example:

In the example I have defined two threads and named them increment and decrement. Increment thread increases the value of shared variable (counter) by the same amount the decrement thread decreases it i.e 5000 times it is increased (which result in 5000 + 0 = 5000) and 5000 times we decrease (which result in 5000 - 5000 = 0).

Program without synchronized keyword:

class SynchronizationDemo {

    public static void main(String[] args){

        Buffer buffer = new Buffer();                   

        MyThread incThread = new MyThread(buffer, "increment");
        MyThread decThread = new MyThread(buffer, "decrement"); 

        incThread.start();
        decThread.start();  
       
        try {
          incThread.join();
          decThread.join();
        }catch(InterruptedException e){ }

        System.out.println("Final counter: "+buffer.getCounter());
    }
}

class Buffer {
    private int counter = 0; 
    public void inc() { counter++; }
    public void dec() { counter--; } 
    public int getCounter() { return counter; }
}

class MyThread extends Thread {

    private String name;
    private Buffer buffer;

    public MyThread (Buffer aBuffer, String aName) {            
        buffer = aBuffer; 
        name = aName; 
    }

    public void run(){
        for (int i = 0; i <= 5000; i++){
            if (name.equals("increment"))
                buffer.inc();
            else
                buffer.dec();                           
        }
    }
}

If we run the above program we expect value of buffer to be same since incrementing and decrementing the buffer by same amount would result in initial value we started with right ?. Lets see the output:

As you can see no matter how many times we run the program we get a different result reason being each thread manipulated the counter at the same time. If we could manage to let the one thread to first increment the shared variable and then second to decrement it or vice versa we will then get the right result that is exactly what can be done with synchronized keyword by just adding synchronized keyword before the inc and dec methods of Buffer like this:

Program with synchronized keyword:

// rest of the code

class Buffer {
    private int counter = 0; 
    // added synchronized keyword to let only one thread
    // be it inc or dec thread to manipulate data at a time
    public synchronized void inc() { counter++; }
    public synchronized void dec() { counter--; } 
    public int getCounter() { return counter; }
}

// rest of the code

and the output:

no matter how many times we run it we get the same output as 0

The synchronized keyword causes a thread to obtain a lock when entering the method, so that only one thread can execute the method at the same time (for the given object instance, unless it is a static method).

This is frequently called making the class thread-safe, but I would say this is a euphemism. While it is true that synchronization protects the internal state of the Vector from getting corrupted, this does not usually help the user of Vector much.

Consider this:

 if (vector.isEmpty()){
     vector.add(data);
 }

Even though the methods involved are synchronized, because they are being locked and unlocked individually, two unfortunately timed threads can create a vector with two elements.

So in effect, you have to synchronize in your application code as well.

Because method-level synchronization is a) expensive when you don't need it and b) insufficient when you need synchronization, there are now un-synchronized replacements (ArrayList in the case of Vector).

More recently, the concurrency package has been released, with a number of clever utilities that take care of multi-threading issues.

Well, I think we had enough of theoretical explanations, so consider this code

public class SOP {
    public static void print(String s) {
        System.out.println(s+"\n");
    }
}

public class TestThread extends Thread {
    String name;
    TheDemo theDemo;
    public TestThread(String name,TheDemo theDemo) {
        this.theDemo = theDemo;
        this.name = name;
        start();
    }
    @Override
    public void run() {
        theDemo.test(name);
    }
}

public class TheDemo {
    public synchronized void test(String name) {
        for(int i=0;i<10;i++) {
            SOP.print(name + " :: "+i);
            try{
                Thread.sleep(500);
            } catch (Exception e) {
                SOP.print(e.getMessage());
            }
        }
    }
    public static void main(String[] args) {
        TheDemo theDemo = new TheDemo();
        new TestThread("THREAD 1",theDemo);
        new TestThread("THREAD 2",theDemo);
        new TestThread("THREAD 3",theDemo);
    }
}

Note: synchronized blocks the next thread's call to method test() as long as the previous thread's execution is not finished. Threads can access this method one at a time. Without synchronized all threads can access this method simultaneously.

When a thread calls the synchronized method 'test' of the object (here object is an instance of 'TheDemo' class) it acquires the lock of that object, any new thread cannot call ANY synchronized method of the same object as long as previous thread which had acquired the lock does not release the lock.

Similar thing happens when any static synchronized method of the class is called. The thread acquires the lock associated with the class(in this case any non static synchronized method of an instance of that class can be called by any thread because that object level lock is still available). Any other thread will not be able to call any static synchronized method of the class as long as the class level lock is not released by the thread which currently holds the lock.

Output with synchronised

THREAD 1 :: 0
THREAD 1 :: 1
THREAD 1 :: 2
THREAD 1 :: 3
THREAD 1 :: 4
THREAD 1 :: 5
THREAD 1 :: 6
THREAD 1 :: 7
THREAD 1 :: 8
THREAD 1 :: 9
THREAD 3 :: 0
THREAD 3 :: 1
THREAD 3 :: 2
THREAD 3 :: 3
THREAD 3 :: 4
THREAD 3 :: 5
THREAD 3 :: 6
THREAD 3 :: 7
THREAD 3 :: 8
THREAD 3 :: 9
THREAD 2 :: 0
THREAD 2 :: 1
THREAD 2 :: 2
THREAD 2 :: 3
THREAD 2 :: 4
THREAD 2 :: 5
THREAD 2 :: 6
THREAD 2 :: 7
THREAD 2 :: 8
THREAD 2 :: 9

Output without synchronized

THREAD 1 :: 0
THREAD 2 :: 0
THREAD 3 :: 0
THREAD 1 :: 1
THREAD 2 :: 1
THREAD 3 :: 1
THREAD 1 :: 2
THREAD 2 :: 2
THREAD 3 :: 2
THREAD 1 :: 3
THREAD 2 :: 3
THREAD 3 :: 3
THREAD 1 :: 4
THREAD 2 :: 4
THREAD 3 :: 4
THREAD 1 :: 5
THREAD 2 :: 5
THREAD 3 :: 5
THREAD 1 :: 6
THREAD 2 :: 6
THREAD 3 :: 6
THREAD 1 :: 7
THREAD 2 :: 7
THREAD 3 :: 7
THREAD 1 :: 8
THREAD 2 :: 8
THREAD 3 :: 8
THREAD 1 :: 9
THREAD 2 :: 9
THREAD 3 :: 9

Here is an explanation from The Java Tutorials.

Consider the following code:

public class SynchronizedCounter {
    private int c = 0;

    public synchronized void increment() {
        c++;
    }

    public synchronized void decrement() {
        c--;
    }

    public synchronized int value() {
        return c;
    }
}

if count is an instance of SynchronizedCounter, then 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.
• Second, when a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object. This guarantees that changes to the state of the object are visible to all threads.

Think of it as a kind of turnstile like you might find at a football ground. There are parallel steams of people wanting to get in but at the turnstile they are 'synchronised'. Only one person at a time can get through. All those wanting to get through will do, but they may have to wait until they can go through.

Overview

Synchronized keyword in Java has to do with thread-safety, that is, when multiple threads read or write the same variable.
This can happen directly (by accessing the same variable) or indirectly (by using a class that uses another class that accesses the same variable).

The synchronized keyword is used to define a block of code where multiple threads can access the same variable in a safe way.

Deeper

Syntax-wise the synchronized keyword takes an Object as it's parameter (called a lock object), which is then followed by a { block of code }.

• When execution encounters this keyword, the current thread tries to "lock/acquire/own" (take your pick) the lock object and execute the associated block of code after the lock has been acquired.

• Any writes to variables inside the synchronized code block are guaranteed to be visible to every other thread that similarly executes code inside a synchronized code block using the same lock object.

• Only one thread at a time can hold the lock, during which time all other threads trying to acquire the same lock object will wait (pause their execution). The lock will be released when execution exits the synchronized code block.

Synchronized methods:

Adding synchronized keyword to a method definition is equal to the entire method body being wrapped in a synchronized code block with the lock object being this (for instance methods) and ClassInQuestion.getClass() (for class methods).

- Instance method is a method which does not have static keyword.
- Class method is a method which has static keyword.

Technical

Without synchronization, it is not guaranteed in which order the reads and writes happen, possibly leaving the variable with garbage.
(For example a variable could end up with half of the bits written by one thread and half of the bits written by another thread, leaving the variable in a state that neither of the threads tried to write, but a combined mess of both.)

It is not enough to complete a write operation in a thread before (wall-clock time) another thread reads it, because hardware could have cached the value of the variable, and the reading thread would see the cached value instead of what was written to it.

Conclusion

Thus in Java's case, you have to follow the Java Memory Model to ensure that threading errors do not happen.
In other words: Use synchronization, atomic operations or classes that use them for you under the hoods.

Sources

http://docs.oracle.com/javase/specs/jls/se8/html/index.html
Java® Language Specification, 2015-02-13

synchronized simple means no two threads can access the block/method simultaneously. When we say any block/method of a class is synchronized it means only one thread can access them at a time. Internally the thread which tries to access it first take a lock on that object and as long as this lock is not available no other thread can access any of the synchronized methods/blocks of that instance of the class.

Note another thread can access a method of the same object which is not defined to be synchronized. A thread can release the lock by calling

Object.wait()

Synchronized normal method equivalent to Synchronized statement (use this)

class A {
    public synchronized void methodA() {
        // all function code
    }

    equivalent to

    public void methodA() {
        synchronized(this) {
             // all function code
        }
    } 
}

Synchronized static method equivalent to Synchronized statement (use class)

class A {
    public static synchronized void methodA() {
        // all function code
    }

    equivalent to

    public void methodA() {
        synchronized(A.class) {
             // all function code
        }
    } 
}

Synchronized statement (using variable)

class A {
    private Object lock1 = new Object();

    public void methodA() {
        synchronized(lock1 ) {
             // all function code
        }
    } 
}

For synchronized, we have both Synchronized Methods and Synchronized Statements. However, Synchronized Methods is similar to Synchronized Statements so we just need to understand Synchronized Statements.

=> Basically, we will have

synchronized(object or class) { // object/class use to provides the intrinsic lock
   // code 
}

Here is 2 think that help understanding synchronized

• Every object/class have an intrinsic lock associated with it.
• When a thread invokes a synchronized statement, it automatically acquires the intrinsic lock for that synchronized statement's object and releases it when the method returns. As long as a thread owns an intrinsic lock, NO other thread can acquire the SAME lock => thread safe.

=> When a thread A invokes synchronized(this){// code 1} => all the block code (inside class) where have synchronized(this) and all synchronized normal method (inside class) is locked because SAME lock. It will execute after thread A unlock ("// code 1" finished).

This behavior is similar to synchronized(a variable){// code 1} or synchronized(class).

SAME LOCK => lock (not depend on which method? or which statements?)

Use synchronized method or synchronized statements?

I prefer synchronized statements because it is more extendable. Example, in future, you only need synchronized a part of method. Example, you have 2 synchronized method and it don't have any relevant to each other, however when a thread run a method, it will block the other method (it can prevent by use synchronized(a variable)).

However, apply synchronized method is simple and the code look simple. For some class, there only 1 synchronized method, or all synchronized methods in the class in relevant to each other => we can use synchronized method to make code shorter and easy to understand

Note

(it not relevant to much to synchronized, it is the different between object and class or none-static and static).

• When you use synchronized or normal method or synchronized(this) or synchronized(non-static variable) it will synchronized base on each object instance.
• When you use synchronized or static method or synchronized(class) or synchronized(static variable) it will synchronized base on class

Reference

https://docs.oracle.com/javase/tutorial/essential/concurrency/syncmeth.html https://docs.oracle.com/javase/tutorial/essential/concurrency/locksync.html

Hope it help

synchronized is a keyword in Java which is used to make happens before relationship in multithreading environment to avoid memory inconsistency and thread interference error.

volatile^[About] => synchronized

synchronized block in Java is a monitor in multithreading. synchronized block with the same object/class can be executed by only single thread, all others are waiting. It can help with race condition situation when several threads try to update the same variable.

Java 5 extended synchronized by supporting happens-before^[About]

An unlock (synchronized block or method exit) of a monitor happens-before every subsequent lock (synchronized block or method entry) of that same monitor.

The next step is java.util.concurrent

What is the synchronized keyword?

Threads communicate primarily by sharing access to fields and the objects reference fields refer to. This form of communication is extremely efficient, but makes two kinds of errors possible: thread interference and memory consistency errors. The tool needed to prevent these errors is synchronization.

Synchronized blocks or methods prevents thread interference and make sure that data is consistent. At any point of time, only one thread can access a synchronized block or method (critical section) by acquiring a lock. Other thread(s) will wait for release of lock to access critical section.

When are methods synchronized?

Methods are synchronized when you add synchronized to method definition or declaration. You can also synchronize a particular block of code with-in a method.

What does it mean pro grammatically and logically?

It means that only one thread can access critical section by acquiring a lock. Unless this thread release this lock, all other thread(s) will have to wait to acquire a lock. They don't have access to enter critical section with out acquiring lock.

This can't be done with a magic. It's programmer responsibility to identify critical section(s) in application and guard it accordingly. Java provides a framework to guard your application, but where and what all sections to be guarded is the responsibility of programmer.

More details from java documentation page

Intrinsic Locks and Synchronization:

Synchronization is built around an internal entity known as the intrinsic lock or monitor lock. Intrinsic locks play a role in both aspects of synchronization: enforcing exclusive access to an object's state and establishing happens-before relationships that are essential to visibility.

Every object has an intrinsic lock associated with it. By convention, a thread that needs exclusive and consistent access to an object's fields has to acquire the object's intrinsic lock before accessing them, and then release the intrinsic lock when it's done with them.

A thread is said to own the intrinsic lock between the time it has acquired the lock and released the lock. As long as a thread owns an intrinsic lock, no other thread can acquire the same lock. The other thread will block when it attempts to acquire the lock.

When a thread releases an intrinsic lock, a happens-before relationship is established between that action and any subsequent acquisition of the same lock.

Making 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.

Second, when a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object.

This guarantees that changes to the state of the object are visible to all threads.

Look for other alternatives to synchronization in :

Avoid synchronized(this) in Java?