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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,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:

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

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:

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

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

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:

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

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

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

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:

技术分享
SyncThread1:0SyncThread1:1SyncThread1:2SyncThread1:3SyncThread1:4SyncThread2:5SyncThread2:6SyncThread2:7SyncThread2:8SyncThread2:9
View Code

 

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

Java Synchronized的用法