英文:
Increase a Counter using 2 threads
问题
我试图通过使用2个线程将初始为0的计数器增加到1000。以下是我的代码：
class CounterVar {
private int val;
CounterVar() {
val = 0;
}
public void increment() {
Object obj = new Object();
synchronized(obj) {
val++;
}
}
public int getVal() {
return val;
}
}
public class Counter {
public static void main(String[] args) throws InterruptedException {
CounterVar counter = new CounterVar();
Thread t1 = new Thread(() -> {
for (int i = 0; i < 500; i++)
counter.increment();
});
Thread t2 = new Thread(() -> {
for (int i = 0; i < 500; i++)
counter.increment();
});
t1.start();
t2.start();
t1.join();
t2.join();
System.out.println(counter.getVal());
}
}
我在这里无法获得输出为1000，它总是小于1000。我已经实现了同步，但不明白问题出在哪里。同时，我想澄清一下，当线程1增加计数器时，是否会让线程2陷入饥饿状态。提前谢谢。
英文:
I am trying to increase a counter which is initially 0,
to 1000 using 2 threads. Here is my code
class CounterVar {
private int val;
CounterVar() {
val = 0;
}
public void increment() {
Object obj = new Object();
synchronized(obj) {
val++;
}
}
public int getVal() {
return val;
}
}
public class Counter {
public static void main(String[] args) throws InterruptedException {
CounterVar counter = new CounterVar();
Thread t1 = new Thread(() - > {
for (int i = 0; i < 500; i++)
counter.increment();
});
Thread t2 = new Thread(() - > {
for (int i = 0; i < 500; i++)
counter.increment();
});
t1.start();
t2.start();
t1.join();
t2.join();
System.out.println(counter.getVal());
}
}
I am not getting output as 1000 here, its always less. I have implemented synchronization and dont understand what is wrong here.
Also wanted to clarify if I am starving thread2 while thread1 is incrementing the counter.
Thanks in Advance.
答案1
得分: 8
你没有正确同步线程。要同步多个线程，必须使用一个共同的对象作为同步机制，但你为每个线程创建了一个新对象。尝试这样做：
class CounterVar {
private int val;
private final Object obj = new Object();
public void increment() {
synchronized(obj) {
val++;
}
}
}
或者，更简单地去掉 obj：
public synchronized void increment() {
...
}
英文:
You are not synchronizing the threads correctly. To synchronize multiple threads, you have to use a common object as the synchronization mechanism, but you are creating a new object for each thread. Instead, try this:
class CounterVar {
private int val;
private final Object obj = new Object();
public void increment() {
synchronized(obj) {
val++;
}
}
Or, simpler without the obj:
public synchronized void increment() {
...
}
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