Synchronized的意思是，如果在特定对象上使用Synchronized块，那么与单个对象关联的多个线程可以防止脏读写。为了让你更清楚，让我们举个例子:

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();
    }
}

我们已经创建了两个MyRunnable类对象，runnable1与线程1和线程3共享，而runnable2仅与线程2共享。 现在，当t1和t3启动时没有使用synchronized, PFB输出表明线程1和线程3同时影响var值，而对于线程2,var有自己的内存。

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

使用Synchronzied，线程3在所有场景中等待线程1完成。有两个锁，一个在runnable1上，由线程1和线程3共享，另一个在runnable2上，仅由线程2共享。

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

好了，我认为我们已经有了足够多的理论解释，所以考虑一下这段代码

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);
    }
}

注意:synchronized会阻塞下一个线程对test()方法的调用，只要前一个线程的执行没有完成。线程一次只能访问一个方法。如果没有同步，所有线程都可以同时访问这个方法。

当一个线程调用对象的同步方法'test'时(这里的对象是'TheDemo'类的一个实例)，它获得了该对象的锁，任何新的线程都不能调用同一对象的任何同步方法，只要之前获得锁的线程没有释放锁。

当调用类的任何静态同步方法时，也会发生类似的事情。线程获得与类关联的锁(在这种情况下，该类实例的任何非静态同步方法都可以被任何线程调用，因为对象级锁仍然可用)。只要当前持有类级锁的线程没有释放类级锁，任何其他线程都不能调用类的任何静态同步方法。

输出同步

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

输出未同步

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

以我的理解，同步基本上意味着编译器编写一个监视器。进入并监控。在方法周围退出。因此，它可能是线程安全的，这取决于它是如何使用的(我的意思是，你可以编写一个具有同步方法的对象，它不是线程安全的，这取决于你的类做什么)。

下面是来自Java教程的解释。

考虑下面的代码:

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.

在java中，为了防止多个线程操纵一个共享变量，我们使用synchronized关键字。让我们通过下面的例子来理解它:

在这个例子中，我定义了两个线程，并将它们命名为increment和decincrement。增量线程增加共享变量(计数器)的值，增量线程减少共享变量(计数器)的值，即增加5000次(结果是5000 + 0 = 5000)，减少5000次(结果是5000 - 5000 = 0)。

没有同步关键字的程序:

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();                           
        }
    }
}

如果我们运行上面的程序，我们期望缓冲区的值是相同的，因为缓冲区的增量和减量相同，将会得到我们开始时的初始值，对吗?让我们看看输出:

正如你所看到的，无论我们运行程序多少次，我们都会得到不同的结果，因为每个线程都在同一时间操纵计数器。如果我们能设法让一个线程先增加共享变量，然后再减少它，反之亦然，我们就会得到正确的结果，这正是synchronized关键字所能做到的，只需在Buffer的inc和dec方法之前添加synchronized关键字，如下所示:

关键字为synchronized的程序:

// 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

输出:

不管我们运行多少次，我们得到的输出都是0

你可以把它想象成一个转门，就像你在足球场看到的那样。有平行的人群想要进入，但在旋转门他们是“同步的”。一次只能有一个人通过。所有想要通过的人都可以，但他们可能要等到他们能够通过。