Java Synchronized的用法

synchronized是Java中的关键字，是一种同步锁。它修饰的对象有以下几种： 
1. 修饰一个代码块，被修饰的代码块称为同步语句块，其作用的范围是大括号{}括起来的代码，作用的对象是调用这个代码块的对象； 
2. 修饰一个方法，被修饰的方法称为同步方法，其作用的范围是整个方法，作用的对象是调用这个方法的对象； 
3. 修改一个静态的方法，其作用的范围是整个静态方法，作用的对象是这个类的所有对象； 
4. 修改一个类，其作用的范围是synchronized后面括号括起来的部分，作用主的对象是这个类的所有对象。

修饰代码块

package com.qhong;

public class Main {
    public static void main(String[] args) {
        SyncThread syncThread = new SyncThread();
        Thread thread1 = new Thread(syncThread, "SyncThread1");
        Thread thread2 = new Thread(syncThread, "SyncThread2");
        thread1.start();
        thread2.start();
    }
}

class SyncThread implements Runnable {
    private static int count;
    public SyncThread() {
        count = 0;
    }
    public  void run() {
        synchronized(this) {
            for (int i = 0; i < 5; i++) {
                try {
                    System.out.println(Thread.currentThread().getName() + ":" + (count++));
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
    public int getCount() {
        return count;
    }
}

结果：

SyncThread1:0
SyncThread1:1
SyncThread1:2
SyncThread1:3
SyncThread1:4
SyncThread2:5
SyncThread2:6
SyncThread2:7
SyncThread2:8
SyncThread2:9

当两个并发线程(thread1和thread2)访问同一个对象(syncThread)中的synchronized代码块时，

在同一时刻只能有一个线程得到执行，另一个线程受阻塞，必须等待当前线程执行完这个代码块以后才能执行该代码块。

Thread1和thread2是互斥的，因为在执行synchronized代码块时会锁定当前的对象，只有执行完该代码块才能释放该对象锁，下一个线程才能执行并锁定该对象。 
我们再把SyncThread的调用稍微改一下：

package com.qhong;

public class Main {
    public static void main(String[] args) {
        Thread thread1 = new Thread(new SyncThread(), "SyncThread1");
        Thread thread2 = new Thread(new SyncThread(), "SyncThread2");
        thread1.start();
        thread2.start();
    }
}

class SyncThread implements Runnable {
    private static int count;
    public SyncThread() {
        count = 0;
    }
    public  void run() {
        synchronized(this) {
            for (int i = 0; i < 5; i++) {
                try {
                    System.out.println(Thread.currentThread().getName() + ":" + (count++));
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
    public int getCount() {
        return count;
    }
}

Result:

SyncThread1:0
SyncThread2:1
SyncThread2:2
SyncThread1:3
SyncThread1:4
SyncThread2:5
SyncThread1:6
SyncThread2:6
SyncThread2:7
SyncThread1:7

======================================================

修饰对象

package com.qhong;

public class Main {
    public static void main(String[] args) {
        Account account = new Account("zhang san", 10000.0f);
        AccountOperator accountOperator = new AccountOperator(account);

        final int THREAD_NUM = 5;
        Thread threads[] = new Thread[THREAD_NUM];
        for (int i = 0; i < THREAD_NUM; i ++) {
            threads[i] = new Thread(accountOperator, "Thread" + i);
            threads[i].start();
        }
    }
}

/**
 * 银行账户类
 */
class Account {
    String name;
    float amount;

    public Account(String name, float amount) {
        this.name = name;
        this.amount = amount;
    }
    //存钱
    public  void deposit(float amt) {
        amount += amt;
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
    //取钱
    public  void withdraw(float amt) {
        amount -= amt;
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public float getBalance() {
        return amount;
    }
}

/**
 * 账户操作类
 */
class AccountOperator implements Runnable{
    private Account account;
    public AccountOperator(Account account) {
        this.account = account;
    }

    public void run() {
        synchronized (account) {
            account.deposit(500);
            account.withdraw(500);
            System.out.println(Thread.currentThread().getName() + ":" + account.getBalance());
        }
    }
}

Result:

Thread1:10000.0
Thread4:10000.0
Thread3:10000.0
Thread2:10000.0
Thread0:10000.0

当没有明确的对象作为锁，只是想让一段代码同步时，可以创建一个特殊的对象来充当锁：

 private byte[] lock = new byte[0];  // 特殊的instance变量
   public void method()
   {
      synchronized(lock) {
         // todo 同步代码块
      }
   }

=====================================================================

修饰一个方法：

======================================================================

修饰一个静态方法：

package com.qhong;

public class Main {
    public static void main(String[] args) {
        SyncThread syncThread1 = new SyncThread();
        SyncThread syncThread2 = new SyncThread();
        Thread thread1 = new Thread(syncThread1, "SyncThread1");
        Thread thread2 = new Thread(syncThread2, "SyncThread2");
        thread1.start();
        thread2.start();
    }
}
class SyncThread implements Runnable {
    private static int count;

    public SyncThread() {
        count = 0;
    }

    public synchronized static void method() {
        for (int i = 0; i < 5; i ++) {
            try {
                System.out.println(Thread.currentThread().getName() + ":" + (count++));
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public synchronized void run() {
        method();
    }
}

Result:

SyncThread2:0
SyncThread2:1
SyncThread2:2
SyncThread2:3
SyncThread2:4
SyncThread1:5
SyncThread1:6
SyncThread1:7
SyncThread1:8
SyncThread1:9

============================================================================

修饰一个类：

package com.qhong;

public class Main {
    public static void main(String[] args) {
        SyncThread syncThread1 = new SyncThread();
        SyncThread syncThread2 = new SyncThread();
        Thread thread1 = new Thread(syncThread1, "SyncThread1");
        Thread thread2 = new Thread(syncThread2, "SyncThread2");
        thread1.start();
        thread2.start();
    }
}
class SyncThread implements Runnable {
    private static int count;

    public SyncThread() {
        count = 0;
    }

    public static void method() {
        synchronized(SyncThread.class) {
            for (int i = 0; i < 5; i ++) {
                try {
                    System.out.println(Thread.currentThread().getName() + ":" + (count++));
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    public synchronized void run() {
        method();
    }
}

Result:

SyncThread1:0
SyncThread1:1
SyncThread1:2
SyncThread1:3
SyncThread1:4
SyncThread2:5
SyncThread2:6
SyncThread2:7
SyncThread2:8
SyncThread2:9

http://blog.csdn.net/luoweifu/article/details/46613015