synchronized常见用法解析及示例

synchronized作用：
保证代码执行的原子性；
保证可见性（与volatile作用相同）

JAVA中synchronized关键字能够作为函数的修饰符，也可作为函数内的语句，也就是平时说的同步方法和同步语句块。
假如再细的分类，synchronized可作用于instance变量、object reference（对象引用，例如this）、static函数和class literals(类名称字面常量)身上。
下面讨论synchronized用到不同地方对代码产生的影响：

1．
对于非static的情况，synchronized是对象级别的，其实质是将synchronized作用于对象引用（object reference）上，即拿到p1对象锁的线程，对p1的fun()方法有同步互斥作用，不同的对象之间坚持“和平共处”。
假设P1、P2是同一个类的不同对象，这个类中定义了以下几种情况的同步块或同步方法，P1、P2就都能够调用他们。
(1)把synchronized当作函数修饰符时，示例代码如下：
public synchronized void method(){
//….
}
这也就是同步方法，那这时synchronized锁定的是哪个对象呢？他锁定的是调用这个同步方法对象。也就是说，当一个对象P1在不同的线程中执行这个同步方法时，他们之间会形成互斥，达到同步的效果。但是这个对象所属的Class所产生的另一对象P2却能够任意调用这个被加了synchronized关键字的方法。上边的示例代码等同于如下代码：
public void method()
{
synchronized (this) // (1)
{
//…..
}
}
(1)处的this指的是什么呢？他指的就是调用这个方法的对象，如P1。可见，同步方法实质是将synchronized作用于Object Reference。那个拿到了P1对象锁的线程，才能够调用P1的同步方法，而对P2而言，P1这个锁和他毫不相干，代码也可能在这种情形下摆脱同步机制的控制，造成数据混乱，譬如同时操作静态变量时。
(2)．同步块，示例代码如下：
public void method(SomeObject so) {
synchronized(so)
{
//…..
}
}
这时，锁就是so这个对象，谁拿到这个锁谁就能够运行他所控制的那段代码。当有一个明确的对象作为锁时，就能够这样写代码，但当没有明确的对象作为锁，只是想让一段代码同步时，能够创建一个特别的instance变量（它得是个对象）来充当锁：
class Foo implements Runnable
{
private byte[] lock = new byte[0]; // 特别的instance变量
Public void method()
{
synchronized(lock) { //… }
}
//…..
}
注：零长度的byte数组对象创建起来将比任何对象都经济。查看编译后的字节码：生成零长度的byte[]对象只需3条操作码，而Object lock = new Object()则需要7行操作码。
2．
如果方法用static修饰，synchronized的作用范围就是class一级的，它对类的所有对象起作用。
将synchronized作用于static 函数,，示例代码如下：
Class Foo
{
public static synchronized void method1() // 同步的static 函数
{
//….
}
public void method2()
{
synchronized(Foo.class) // class literal(类名称字面常量)
//请注意，Foo.class也是一个对象，类型是Class，在一个ClassLoader里，它是唯一的。
}
}
代码中的method2()方法是把class literal作为锁的情况，他和同步的static函数产生的效果是相同的，
取得的锁很特别，是当前调用这个方法的对象所属的类（Class类，而不再是由这个Class产生的某个具体对象了）。

http://www.strutshome.com/index.php/archives/495

http://www.2cto.com/kf/201206/134899.html

http://tech.it168.com/j/2008-01-30/200801302324557.shtml

1.
A: synchronized static是某个类的范围，synchronized static cSync{}防止多个线程同时访问这个 类中的synchronized static 方法。它可以对类的所有对象实例起作用。
B: synchronized 是某实例的范围，synchronized isSync(){}防止多个线程同时访问这个实例中的synchronized 方法。

2.
synchronized方法与synchronized代码快的区别
synchronized methods(){} 与synchronized（this）{}之间没有什么区别，只是 synchronized methods(){} 便于阅读理解，
而synchronized（this）{}可以更精确的控制冲突限制访问区域，有时候表现更高效率。

3.synchronized关键字是不能继承的，即，父类的synchronized方法在子类中不是synchronized，必须要重新的显式的声明为synchronized才行。

4.实现同步需要很大的系统开销，导致延迟等待，甚至可能造成死锁，所以在非多线程情况下不要使用。

http://www.cnblogs.com/shipengzhi/articles/2223100.html

代码示例：

synchronized使用在方法中是锁住当前对象，此对象中其它synchrinized方法都会阻塞。

synchronized(this)示例：

import java.util.concurrent.TimeUnit;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/*2015-4-29*/
public class SynchronizedTest {
    public static void main(String[] args) throws InterruptedException {
        Task task=new Task(true);
        Thread readThread=new Thread(task, "Reader");
        readThread.start();
        TimeUnit.SECONDS.sleep(2);

        task.setRead(false);
        Thread writeThread=new Thread(task, "Writer");
        writeThread.start();
    }
}

class Task implements Runnable{
    private boolean isRead;
    public Task(boolean isRead) {
        this.isRead=isRead;
    }
    private Logger logger=LoggerFactory.getLogger(Task.class);
    @Override
    public void run() {
        try {
            if (isRead) {
                read();
            }else {
                write();
            }

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public void read() throws InterruptedException{
        synchronized (this) {
            logger.info("Enter read() synchronized");
            TimeUnit.SECONDS.sleep(20);
            logger.info("Ready to leave read()  synchronized");
        }
    }

    public void write(){
        logger.info("enter write()");
        synchronized (this) {
            logger.info("enter  write() synchronized");
            logger.info("Ready to leave write() synchronied");
        }
        logger.info("Ready to leave write()");
    }

    public void setRead(boolean isRead) {
        this.isRead = isRead;
    }

}

输出：

[2015-04-30 06:30:21,290] [Reader] INFO - Enter read() synchronized
[2015-04-30 06:30:23,290] [Writer] INFO - enter write()
[2015-04-30 06:30:41,290] [Reader] INFO - Ready to leave read()  synchronized
[2015-04-30 06:30:41,290] [Writer] INFO - enter  write() synchronized
[2015-04-30 06:30:41,290] [Writer] INFO - Ready to leave write() synchronied
[2015-04-30 06:30:41,290] [Writer] INFO - Ready to leave write()

synchronized method_name示例：

package thread.synchronizedTest;

import java.util.concurrent.TimeUnit;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class SynchronizedMethodTest {
    public static void main(String[] args) throws InterruptedException {
        Task3 task3 = new Task3(true);
        Thread readThread = new Thread(task3, "Reader");
        readThread.start();
        TimeUnit.SECONDS.sleep(2);

        task3.setRead(false);
        Thread writeThread = new Thread(task3, "Writer");
        writeThread.start();
    }
}

class Task3 implements Runnable {
    private boolean isRead;

    public Task3(boolean isRead) {
        this.isRead = isRead;
    }

    private Logger logger = LoggerFactory.getLogger(Task3.class);

    @Override
    public void run() {
        try {
            if (isRead) {
                read();
            } else {
                write();
            }

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public synchronized void read() throws InterruptedException {
        logger.info("Enter read() synchronized");
        TimeUnit.SECONDS.sleep(20);
        logger.info("Ready to leave read()  synchronized");
    }

    public synchronized void write() {
        logger.info("enter write()");
        // synchronized (this) {
        logger.info("enter  write() synchronized");
        logger.info("Ready to leave write() synchronied");
        // }
        logger.info("Ready to leave write()");
    }

    public void setRead(boolean isRead) {
        this.isRead = isRead;
    }

}

输出：

[2015-04-30 07:02:04,583] [Reader] INFO - Enter read() synchronized
[2015-04-30 07:02:24,583] [Reader] INFO - Ready to leave read()  synchronized
[2015-04-30 07:02:24,583] [Writer] INFO - enter write()
[2015-04-30 07:02:24,583] [Writer] INFO - enter  write() synchronized
[2015-04-30 07:02:24,583] [Writer] INFO - Ready to leave write() synchronied
[2015-04-30 07:02:24,583] [Writer] INFO - Ready to leave write()

使用一个声明的变量作为锁时，可以让锁的粒度更小，如果合理使用会提高程序执行效率：

package thread.synchronizedTest;

import java.util.concurrent.TimeUnit;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/*2015-4-29*/
public class SynchronizedLockObjectTest {
    public static void main(String[] args) throws InterruptedException {
        Task2 task2=new Task2(true);
        Thread readThread=new Thread(task2, "Reader");
        readThread.start();
        TimeUnit.SECONDS.sleep(2);

        task2.setRead(false);
        Thread writeThread=new Thread(task2, "Writer");
        writeThread.start();
    }
}

class Task2 implements Runnable{

    private byte[] readLock = new byte[0]; // 特别的instance变量
    private byte[] writeLock = new byte[0]; // 特别的instance变量

    private boolean isRead;
    public Task2(boolean isRead) {
        this.isRead=isRead;
    }
    private Logger logger=LoggerFactory.getLogger(Task2.class);
    @Override
    public void run() {
        try {
            if (isRead) {
                read();
            }else {
                write();
            }

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public void read() throws InterruptedException{
        synchronized (readLock) {
            logger.info("Enter read() synchronized");
            TimeUnit.SECONDS.sleep(20);
            logger.info("Ready to leave read()  synchronized");
        }
    }

    public void write(){
        logger.info("enter write()");
        synchronized (writeLock) {
            logger.info("enter  write() synchronized");
            logger.info("Ready to leave write() synchronied");
        }
        logger.info("Ready to leave write()");
    }

    public void setRead(boolean isRead) {
        this.isRead = isRead;
    }

}

输出：

[2015-04-30 06:37:20,148] [Reader] INFO - Enter read() synchronized
[2015-04-30 06:37:22,148] [Writer] INFO - enter write()
[2015-04-30 06:37:22,148] [Writer] INFO - enter  write() synchronized
[2015-04-30 06:37:22,148] [Writer] INFO - Ready to leave write() synchronied
[2015-04-30 06:37:22,148] [Writer] INFO - Ready to leave write()
[2015-04-30 06:37:40,149] [Reader] INFO - Ready to leave read()  synchronized

synchronized(字段)和synchronized(当前对象)在一个实例中同时存在时，两个方法会同时执行，因为synchronized持有的对象不同，一个是当前对象，一个字段对象：

package thread.synchronizedTest;

import java.util.concurrent.TimeUnit;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class SychronizedLockFieldorMethod {
    public static void main(String[] args) throws InterruptedException {
        Task4 task4 = new Task4(true);
        Thread readThread = new Thread(task4, "Reader");
        readThread.start();
        TimeUnit.SECONDS.sleep(2);

        task4.setRead(false);
        Thread writeThread = new Thread(task4, "Writer");
        writeThread.start();
    }
}

class Task4 implements Runnable {

    private byte[] readLock = new byte[0]; // 特别的instance变量
    private byte[] writeLock = new byte[0]; // 特别的instance变量

    private boolean isRead;

    public Task4(boolean isRead) {
        this.isRead = isRead;
    }

    private Logger logger = LoggerFactory.getLogger(Task4.class);

    @Override
    public void run() {
        try {
            if (isRead) {
                read();
            } else {
                write();
            }

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public synchronized void read() throws InterruptedException {
        // synchronized (readLock) {
        logger.info("Enter read() synchronized");
        TimeUnit.SECONDS.sleep(20);
        logger.info("Ready to leave read()  synchronized");
        // }
    }

    public void write() {
        logger.info("enter write()");
        synchronized (writeLock) {
            logger.info("enter  write() synchronized");
            logger.info("Ready to leave write() synchronied");
        }
        logger.info("Ready to leave write()");
    }

    public void setRead(boolean isRead) {
        this.isRead = isRead;
    }

}

输出：

[2015-04-30 06:54:41,294] [Reader] INFO - Enter read() synchronized
[2015-04-30 06:54:43,294] [Writer] INFO - enter write()
[2015-04-30 06:54:43,294] [Writer] INFO - enter  write() synchronized
[2015-04-30 06:54:43,294] [Writer] INFO - Ready to leave write() synchronied
[2015-04-30 06:54:43,294] [Writer] INFO - Ready to leave write()
[2015-04-30 06:55:01,295] [Reader] INFO - Ready to leave read()  synchronized

synchronized如果是一个入参，则不能起到同步的作用。因为这种场景和synchronized(this)的原理是一样的。但每次入参都是不同的对象，这些不同的对象不能形成互斥

import java.util.concurrent.TimeUnit;

public class SynchronizedParameter implements Runnable {
    private static Biz biz = new Biz();

    private String key;

    public SynchronizedParameter(String key) {
        this.key = key;
    }

    public static void main(String[] args) throws InterruptedException {

        String key1 = "key1";
        Thread thread1 = new Thread(new SynchronizedParameter(key1), key1);
        String key2 = "key2";
        Thread thread2 = new Thread(new SynchronizedParameter(key2), key2);
        Thread thread2_bak = new Thread(new SynchronizedParameter(key2), key2 + "bak");

        thread1.start();
        thread2.start();
        thread2_bak.start();

        thread1.join();
        thread2.join();
        thread2_bak.join();

        System.out.println(Thread.currentThread() + "end...");
    }

    @Override
    public void run() {
        biz.handle(key);
    }

    public void setKey(String key) {
        this.key = key;
    }

}

class Biz {

    public void handle(String key) {
        System.out.println(Thread.currentThread() + " " + key + " " + System.currentTimeMillis());
        synchronized (key) {
            try {
                TimeUnit.SECONDS.sleep(1);
                System.out.println(Thread.currentThread() + " " + key + " " + System.currentTimeMillis());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

}

Thread[key1,5,main] key1 1476842469690
Thread[key2,5,main] key2 1476842469691
Thread[key2bak,5,main] key2 1476842469695
Thread[key2,5,main] key2 1476842470691
Thread[key1,5,main] key1 1476842470691
Thread[key2bak,5,main] key2 1476842471691
Thread[main,5,main]end...