多线程同步问题-练习题3-笔记整理

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练习题3：

程序同时启用了4个线程去调用TestDo.doSome()方法，由于TestDo.doSome方法内部的代码是先暂停1秒，
然后在输出以秒为单位的当前时间值，所以，会打印出4个相同的时间值，如下所示：
4:4:1258199615
1:1:1258199615
3:3:1258199615
1:1:1258199615
修改代码，如果有几个线程调用TestDo.doSome方法时，传递进去的key相等，则这几个线程应互斥排队输出结果，
即当有有个线程的key都是“1”时，他们中的一个要比其他线程晚1秒输出结果。

提示：对key相同的线程处理时，使用同一把锁，阻塞

题目代码示例：

public class Test3 extends Thread{private TestDo1 testDo;private String key;private String value;public Test3(String key1,String key2, String value) {this.testDo = TestDo1.getInstance();this.key = key1+key2;this.value = value;}public static void main(String[] args) {Test3 a=new Test3("1","","1");Test3 b=new Test3("2","","2");Test3 c=new Test3("3","","3");Test3 d=new Test3("4","","4");System.out.println("begin:"+System.currentTimeMillis()/1000);a.start();b.start();c.start();d.start();}public void run(){testDo.doSome(key, value);}}class TestDo1{private TestDo1(){}private  static TestDo1 _instance=null;public static TestDo1 getInstance(){if(_instance==null){synchronized (TestDo1.class) {if(_instance==null){ _instance=new TestDo1();}}}return _instance;}public static void doSome(Object key,String value){try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}System.out.println(key+":"+value+":"+System.currentTimeMillis()/1000);}}

修改后代码示例:

public class Test3_1 extends Thread{private TestDo1 testDo;private String key;private String value;public Test3_1(String key1,String key2, String value) {this.testDo = TestDo1.getInstance();this.key = key1+key2;this.value = value;}public static void main(String[] args) {Test3_1 a=new Test3_1("1","","1");Test3_1 b=new Test3_1("2","","2");Test3_1 c=new Test3_1("3","","3");Test3_1 d=new Test3_1("1","","1");System.out.println("begin:"+System.currentTimeMillis()/1000);a.start();b.start();c.start();d.start();}public void run(){testDo.doSome(key, value);}}class TestDo1{private TestDo1(){}private  static TestDo1 _instance=null;public static TestDo1 getInstance(){if(_instance==null){synchronized (TestDo1.class) {if(_instance==null){ _instance=new TestDo1();}}}return _instance;}ArrayList<String> list=new ArrayList<String>();public void doSome(Object key,String value){String str=(String)key;                       //关键点，维护key相同，使用相同的锁阻塞if(list.contains(key)){Iterator iterator = list.iterator();while(iterator.hasNext()){String tmp = (String)iterator.next();if(tmp.equals(key)){str=tmp;}}}else{list.add((String)key);}synchronized (str) {try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}System.out.println(key+":"+value+":"+System.currentTimeMillis()/1000);}}}

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