synchronized(obj) 不行,为什么结果可以是-1或者0? 当我使用 synchronized(this) 时就可以了 那为什么会这样
public class RunnableSales {
public static void main(String[] args) {
Ticket1 t = new Ticket1();
Thread t1 = new Thread(t);
Thread t2 = new Thread(t);
Thread t3 = new Thread(t);
Thread t4 = new Thread(t);
t1.start();
t2.start();
t3.start();
t4.start();
}
}
class Ticket1 implements Runnable {
private int tick = 10;
public void run() {
Object obj = new Object();
while (true) {
synchronized (obj) {
if (tick > 0) {
try {
Thread.sleep(10);
} catch (InterruptedException e) {
}
System.out.println(Thread.currentThread().getName() + "sale:" + tick--);
} else {
break;
}
}
}
}
}
控制台打印是:
Thread-1sale:10
Thread-2sale:10
Thread-3sale:10
Thread-0sale:9
Thread-2sale:8
Thread-3sale:7
Thread-1sale:8
Thread-0sale:8
Thread-1sale:6
Thread-0sale:3
Thread-2sale:5
Thread-3sale:4
Thread-1sale:2
Thread-0sale:1
Thread-2sale:0
Thread-3sale:-1
synchronized(obj) 不行,为什么结果可以是-1或者0? 当我使用 synchronized(this) 时就可以了 那么为什么会这样
最佳答案
查看代码,发现每次调用run()
都会生成一个新的Object obj
,仅用于本次调用的同步。因此,实际上并没有进行同步。修复非常简单:将 Object obj
设为实例字段。您应该将其设置为 final
以禁止重新分配(这可能会干扰同步)。
固定版本看起来像这样:
public class Test {
public static void main(String[] args) {
Ticket1 t = new Ticket1(4);
Thread t1 = new Thread(t);
Thread t2 = new Thread(t);
Thread t3 = new Thread(t);
Thread t4 = new Thread(t);
t1.start();
t2.start();
t3.start();
t4.start();
try{
t1.join();
t2.join();
t3.join();
t4.join();
} catch (InterruptedException e){
}
}
}
class Ticket1 implements Runnable {
private int tick = 10;
private final Object obj = new Object();
public void run() {
while (true) {
synchronized (obj) {
if (tick > 0) {
try {
Thread.sleep(10);
} catch (InterruptedException e) {
}
System.out.println(Thread.currentThread().getName() + "sale:" + tick--);
} else {
break;
}
}
}
}
}
我添加了代码以显式等待线程终止。您应该始终包含此代码以确保所有衍生线程在主线程退出之前完成。输出看起来像这样:
Thread-3sale:10
Thread-3sale:9
Thread-3sale:8
Thread-3sale:7
Thread-3sale:6
Thread-3sale:5
Thread-3sale:4
Thread-3sale:3
Thread-3sale:2
Thread-3sale:1
请注意,最终输出显示值 1
。对于您的程序,这是正确的,因为在 tick
减少到 0
后没有输出。
关于java - synchronized(obj) 不起作用,为什么结果可以是 -1 或 0?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/49957056/