Java中的多线程

摘要: 线程安全和非线程安全

经常听到StringBuffer是线程安全的,String是线程不安全的,可是什么是线程安全呢,什么是线程不安全呢,这些困扰了我们这么久

,在高并发情况下,我们要注意线程安全,全局变量一般都是线程不安全,而局部变量一般都是安全的看代码

package org.thread;

public class Count {

private int num =0;
public void getCount() {

for (int i = 0; i <= 100; i++) {
num += i;
}
System.out.println(Thread.currentThread().getName() + "============"
+ num);
}

}

这个方法是一个计数的方法,我希望他每次返回都是5050

package org.thread;

public class ThreadTest {

public static void main(String[] args) {

Runnable runnable = new Runnable() {

Count count = new Count();
@Override
public void run() {

count.getCount();

}
};

for(int i = 0;i<=100;i++){
new Thread(runnable).start();

}
}

}

这个是线程的测试类,线程启动我希望他每次拿到都是5050

看结果

Thread-0============5050
Thread-2============10100
Thread-4============19336
Thread-9============39536
Thread-8============44586
Thread-6============49636
Thread-11============59736
Thread-3============24386
Thread-1============19336
Thread-15============64786
Thread-7============29436
Thread-12============54686
Thread-5============34486

Thread-0============5050这个
这有这个是我想要的,其他数据都是多余的

那么现在我想得到5050 有几种方式

第一种

public class Count {

public void getCount() {
int num =0;
for (int i = 0; i <= 100; i++) {
num += i;
}
System.out.println(Thread.currentThread().getName() + "============"
+ num);
}

}

把

private int num =0;设置成内部变量也就是在getCount方法中

得到的结果

Thread-0============5050
Thread-1============5050
Thread-6============5050
Thread-2============5050
Thread-3============5050
Thread-10============5050
Thread-8============5050
Thread-7============5050
Thread-5============5050
Thread-4============5050
Thread-9============5050

这个是我想要的

还有一种就是

Runnable runnable = new Runnable() {

@Override
public void run() {
Count count = new Count();
count.getCount();

}
};

把

Count count = new Count();放到run方法中,每次都创建

这种方法消耗内存性能,不提倡

得到的结果

Thread-0============5050
Thread-1============5050
Thread-6============5050
Thread-2============5050
Thread-3============5050
Thread-10============5050
Thread-8============5050
Thread-7============5050
Thread-5============5050
Thread-4============5050
Thread-9============5050