java 多线程锁synchronized

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对代码加锁：1.在方法前加锁。public synchronized void add(String name) {}2.在方法里面加锁。synchronized (this) {}--未加锁package com.lhj.java;public class Test extends Thread{        Timer timer=new Timer();    public static void main(String[] args)  throws Exception {            Test t = new Test();        Thread t1 = new Thread(t);        Thread t2 = new Thread(t);                t1.setName("t1");                t2.setName("t2");                t1.start();                t2.start();    }       public void run() {            timer.add(Thread.currentThread().getName());//把名字传给add打印    }}class Timer{        private static int i=0;        public void add(String name) {                        i++;                        try {                                Thread.sleep(1000);                        } catch (InterruptedException e) {                                e.printStackTrace();                        }                        System.out.println("hello "+name+",i="+i);        }}-----------------------------hello t2,i=2hello t1,i=2原因是：t1add的时候将i变成了1，然后t1睡着了，t2add的时候将i变成了2，然后t1醒来打印的i为2，t2醒来打印的i也是2--在方法前加锁。public synchronized void add(String name) {}package com.lhj.java;public class Test extends Thread{        Timer timer=new Timer();    public static void main(String[] args)  throws Exception {            Test t = new Test();        Thread t1 = new Thread(t);        Thread t2 = new Thread(t);                t1.setName("t1");                t2.setName("t2");                t1.start();                t2.start();    }       public void run() {            timer.add(Thread.currentThread().getName());//把名字传给add打印    }}class Timer{        private static int i=0;        public synchronized void add(String name) {                        i++;                        try {                                Thread.sleep(1000);                        } catch (InterruptedException e) {                                e.printStackTrace();                        }                        System.out.println("hello "+name+",i="+i);        }}------------------------hello t1,i=1hello t2,i=2--在方法里面加锁。synchronized (this) {}package com.lhj.java;public class Test extends Thread{        Timer timer=new Timer();    public static void main(String[] args)  throws Exception {            Test t = new Test();        Thread t1 = new Thread(t);        Thread t2 = new Thread(t);                t1.setName("t1");                t2.setName("t2");                t1.start();                t2.start();    }       public void run() {            timer.add(Thread.currentThread().getName());//把名字传给add打印    }}class Timer{        private static int i=0;        public void add(String name) {                synchronized (this) {                        i++;                        try {                                Thread.sleep(1000);                        } catch (InterruptedException e) {                                e.printStackTrace();                        }                        System.out.println("hello "+name+",i="+i);                }        }}------------------------hello t1,i=1hello t2,i=2
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