为什么 当一个线程在同步块里的时候,另一个线程还可以进入啊?
各位好~~
本人在学习线程的时候碰到一个问题
以下是代码
public class Test implements Runnable{
public static Integer num = 1;
private String name;
public Test(String name){
this.name = name;
}
public void run(){
int i = 0;
try{
while(true){
System.out.println(name + ": outside");
synchronized(num){
System.out.println(name + ": entered");
if(i > 10){
break;
}
i++;
num--;
Thread.sleep(10);
System.out.println(name + ": leaved");
}
}
}catch(Exception exp){
exp.printStackTrace();
}
}
public static void main(String[] args){
Test t1 = new Test("t1");
Test t2 = new Test("t2");
Thread th1 = new Thread(t1);
Thread th2 = new Thread(t2);
th1.start();
th2.start();
}
}
输出结果为
t1: outside
t1: entered
t2: outside
t2: entered
t1: leaved
t1: outside
t1: entered
t2: leaved
t2: outside
t2: entered
t1: leaved
t1: outside
t1: entered
t2: leaved
t2: outside
t2: entered
t1: leaved
t1: outside
t1: entered
t2: leaved
t2: outside
t2: entered
t1: leaved
t1: outside
t1: entered
t2: leaved
t2: outside
t2: entered
t1: leaved
t1: outside
t1: entered
t2: leaved
t2: outside
t2: entered
t1: leaved
t1: outside
t1: entered
t2: leaved
t2: outside
t2: entered
t1: leaved
t1: outside
t1: entered
t2: leaved
t2: outside
t2: entered
t1: leaved
t1: outside
t1: entered
t2: leaved
t2: outside
t2: entered
t1: leaved
t1: outside
t1: entered
t2: leaved
t2: outside
t2: entered
t1: leaved
t1: outside
t1: entered
t2: leaved
t2: outside
t2: entered
t1: leaved
t1: outside
t1: entered
t2: leaved
t2: outside
t2: entered
为什么当t1线程在同步块里的时候,t2也可以进来啊?如果我把 num--; 这句注释掉就没有这个问题
谁能告诉我这是为什么啊???谢谢
[解决办法]
明白了。
注意这里：
num--
这句相当于：
num = num - 1;
这里有自动拆箱和装箱的动作，首先，原来的num被拆成一个一般的int，用它跟1相减，然后其结果再被重新装箱成“另一个”Integer对象赋给静态变量num。
说起来好像挺复杂，但对你的问题而言，不需要知道那么多，只需要明白一点，经过“num--”之后，num变量就不再引用原来的那个对象了，这就使得两个线程完全可能开始在两个不同的对象上进行synchonized，于是它们也就互不干涉，互不影响，这也是为什么注释掉这句就没有问题的原因。
在这种情形下，如果要避免产生这种问题，可以考虑将num定义成static final的，那样的话，就不能进行“--”了（编译会帮你提示错误）。
如果确实需要“--”，就只能靠细心了。不过至少也应记住，自动装箱/拆箱要慎用，否则弄出问题，半天调试不出原因出在哪里。
