四、java多线程核心技术——synchronized同步方法与synchronized同步快

一、synchronized同步方法

论：“线程安全”与“非线程安全”是多线程的经典问题。synchronized（）方法就是解决非线程安全的。

1、方法内的变量为线程安全

public void addI(String username) {try {int num = 0;     \\方法内的变量为线程安全if (username.equals("a")) {num = 100;System.out.println("a set over!");Thread.sleep(2000);} else {num = 200;System.out.println("b set over!");}System.out.println(username + " num=" + num);} catch (InterruptedException e) {// TODO Auto-generated catch block
            e.printStackTrace();}}

2、实例变量非线程安全

public class HasSelfPrivateNum {private int num = 0;    \\实例变量非线程安全public void addI(String username) {try {if (username.equals("a")) {num = 100;System.out.println("a set over!");Thread.sleep(2000);} else {num = 200;System.out.println("b set over!");}System.out.println(username + " num=" + num);} catch (InterruptedException e) {// TODO Auto-generated catch block
            e.printStackTrace();}}}

解决方法：方法前加synchronized关键字。

public class HasSelfPrivateNum {private int num = 0;synchronized public void addI(String username) {..............}}

3、多个对象多个锁

HasSelfPrivateNum.java

public class HasSelfPrivateNum {private int num = 0;synchronized public void addI(String username) {try {if (username.equals("a")) {num = 100;System.out.println("a set over!");Thread.sleep(2000);} else {num = 200;System.out.println("b set over!");}System.out.println(username + " num=" + num);} catch (InterruptedException e) {// TODO Auto-generated catch block
            e.printStackTrace();}}}

ThreadA

    public class ThreadA extends Thread {private HasSelfPrivateNum numRef;public ThreadA(HasSelfPrivateNum numRef) {super();this.numRef = numRef;}@Overridepublic void run() {super.run();numRef.addI("a");}}

ThreadB

public class ThreadB extends Thread {private HasSelfPrivateNum numRef;public ThreadB(HasSelfPrivateNum numRef) {super();this.numRef = numRef;}@Overridepublic void run() {super.run();numRef.addI("b");}}

RUN

public class Run {public static void main(String[] args) {HasSelfPrivateNum numRef1 = new HasSelfPrivateNum();HasSelfPrivateNum numRef2 = new HasSelfPrivateNum();ThreadA athread = new ThreadA(numRef1);athread.start();ThreadB bthread = new ThreadB(numRef2);bthread.start();}}

结果：创建了2个业务实例，产生2个锁，所以运行结果是异步的。同步为synchronized 异步：asynchronized

4、synchronized 锁重入

当一个线程得到一个对象锁时，再次请求该对象锁时是可以再次得到该对象的锁的。继承关系也可重入锁。

当一个线程执行发生异常时，其持有的锁会自动释放。

同步不具有继承性。的在子类方法中添加synchronized的关键字。

public class HasSelfPrivateNum {synchronized public void service1() {System.out.println("service1");service2();}synchronized public void service2() {System.out.println("service2");service3();}synchronized public void service3() {System.out.println("service3");}
}

HasSelfPrivateNum.java

public class mYThread extends Thread {@Overridepublic void run() {HasSelfPrivateNum hspn=new HasSelfPrivateNum();hspn.service1();    }public static void main(String[] args) {mYThread mYThread=new mYThread();mYThread.start();}
}

mYThread.java

二、synchronized同步快

synchronized同步方法持有锁后，如果长时间不释放，那另一个线程就必须长时间等待。。这种情况下synchronized同步快来解决

使用方法如图：

1、synchronized同步快之间具有同步性，当线程访问object的一个 synchronized(this)同步代码块时，其他线程对同一个object对象中所有其他的

synchronized(this)同步代码块的访问会被阻塞。

2、synchronized(this)同步代码块是锁定当前对象的。

1、synchronized同步快解决同步方法效率低得弊端

public class Task {private String getData1;private String getData2;public synchronized void doLongTimeTask() {try {System.out.println("begin task");Thread.sleep(3000);getData1 = "长时间处理任务后从远程返回的值1 threadName="+ Thread.currentThread().getName();getData2 = "长时间处理任务后从远程返回的值2 threadName="+ Thread.currentThread().getName();System.out.println(getData1);System.out.println(getData2);System.out.println("end task");} catch (InterruptedException e) {// TODO Auto-generated catch block
            e.printStackTrace();}}
}

Task

public class MyThread1 extends Thread {private Task task;public MyThread1(Task task) {super();this.task = task;}@Overridepublic void run() {super.run();CommonUtils.beginTime1 = System.currentTimeMillis();task.doLongTimeTask();CommonUtils.endTime1 = System.currentTimeMillis();}}

MyThread1

public class MyThread2 extends Thread {private Task task;public MyThread2(Task task) {super();this.task = task;}@Overridepublic void run() {super.run();CommonUtils.beginTime2 = System.currentTimeMillis();task.doLongTimeTask();CommonUtils.endTime2 = System.currentTimeMillis();}}

MyThread2

public class CommonUtils {public static long beginTime1;public static long endTime1;public static long beginTime2;public static long endTime2;
}

CommonUtils

public class Run {public static void main(String[] args) {Task task = new Task();MyThread1 thread1 = new MyThread1(task);thread1.start();MyThread2 thread2 = new MyThread2(task);thread2.start();try {Thread.sleep(10000);} catch (InterruptedException e) {e.printStackTrace();}long beginTime = CommonUtils.beginTime1;if (CommonUtils.beginTime2 < CommonUtils.beginTime1) {beginTime = CommonUtils.beginTime2;}long endTime = CommonUtils.endTime1;if (CommonUtils.endTime2 > CommonUtils.endTime1) {endTime = CommonUtils.endTime2;}System.out.println("耗时：" + ((endTime - beginTime) / 1000));}
}

Run

修改Task

public class Task {private String getData1;private String getData2;public void doLongTimeTask() {try {System.out.println("begin task");Thread.sleep(3000);String privateGetData1 = "长时间处理任务后从远程返回的值1 threadName="+ Thread.currentThread().getName();String privateGetData2 = "长时间处理任务后从远程返回的值2 threadName="+ Thread.currentThread().getName();synchronized (this) {getData1 = privateGetData1;getData2 = privateGetData2;}System.out.println(getData1);System.out.println(getData2);System.out.println("end task");} catch (InterruptedException e) {// TODO Auto-generated catch block
            e.printStackTrace();}}
}

View Code

转载于:https://www.cnblogs.com/aGboke/p/6523365.html