问题的出现
上代码
public class Num {
private int num;
public int getNum() {
return num;
}
public int add(int num) {
this.num += num;
return this.num;
}
}
测试类
public class ErrorDemo implements Runnable {
private Num num = new Num();
@Override
public void run() {
for(int i = 0; i < 3; i++) {
try {
Thread.sleep(1000);
num.add(10);
System.out.println(Thread.currentThread().getName()+" value is " + num.getNum());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) {
Runnable runnable = new ErrorDemo();
Thread thread = new Thread(runnable, "thread-01");
Thread thread2 = new Thread(runnable, "thread-02");
thread.start();
thread2.start();
}
}
结果
thread-01 value is 20
thread-02 value is 20
thread-02 value is 40
thread-01 value is 40
thread-02 value is 50
thread-01 value is 60
Process finished with exit code 0
这个结果是不准确的。原因是多个线程不加限制的共同操作同一个数据导致。
该怎么解决这个问题呢？
#线程的同步
知道导致问题的原因，解决起来也不是很困难。 我们可以限制：每次只有一个线程可以操作数据 具体实现一般需满足以下2个条件
- 将竞争访问的资源变量设置为private
- 使用关键字synchronized同步修改变量的代码
##同步方法 对ErrorDemo中的run()加上关键字
@Override
public synchronized void run() {
for(int i = 0; i < 3; i++) {
try {
Thread.sleep(1000);
num.add(10);
System.out.println(Thread.currentThread().getName()+" value is " + num.getNum());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
结果：
thread-01 value is 10
thread-01 value is 20
thread-01 value is 30
thread-02 value is 40
thread-02 value is 50
thread-02 value is 60
Process finished with exit code 0
##同步代码块 上面的同步方法，也可以写成
@Override
public void run() {
for(int i = 0; i < 3; i++) {
try {
synchronized (this) {
Thread.sleep(1000);
num.add(10);
System.out.println(Thread.currentThread().getName()+" value is " + num.getNum());
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
注意
- 如果是同步静态方法，代码块中，this应替换成className.class
- java.util.concurrent.locks Lock接口也可完成类似功能，且思路更为清晰。
