Java 多线程数量

在Java并发编程实践一书中有提到，同时并发线程数量等于CPU核心数-1. 这个道理很浅显，就是要确保多个线程可以同时并发从而效率达到最高。今天写了个程序，发现故事的真相不是这样的....

先写一个具有共享资源的类

package com.lenovo.plm.dms.p5;

public class Service {

private long record;

public Service(long record){
this.record = record;
}

synchronized public void addRecord(){

System.out.println(Thread.currentThread().getName() + ":"+ System.currentTimeMillis() +"------------->" + record++);

}

}

然后线程类

package com.lenovo.plm.dms.p5;

public class MyThread extends Thread {

@Override
public void run() {
// TODO Auto-generated method stub
super.run();
Service service = new Service(0);
for(int i = 0;i<1000;i++){
service.addRecord();
try {
Thread.sleep(3000);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}

}

最后执行类：

package com.lenovo.plm.dms.p5;

public class Main {

public static void main(String[] args) {

for(int i = 0;i<40;i++){
MyThread t = new MyThread();
t.start();
}

}

}

这里一共创建了40个线程，看执行结果，比较长：

Thread-0:1455797512547------------->0

Thread-6:1455797512547------------->0

Thread-7:1455797512547------------->0

Thread-9:1455797512547------------->0

Thread-8:1455797512547------------->0

Thread-13:1455797512547------------->0

Thread-4:1455797512547------------->0

Thread-1:1455797512547------------->0

Thread-2:1455797512547------------->0

Thread-3:1455797512547------------->0

Thread-15:1455797512547------------->0

Thread-12:1455797512547------------->0

Thread-11:1455797512547------------->0

Thread-5:1455797512547------------->0

Thread-14:1455797512547------------->0

Thread-16:1455797512547------------->0

Thread-17:1455797512547------------->0

Thread-10:1455797512547------------->0

Thread-18:1455797512547------------->0

Thread-19:1455797512547------------->0

Thread-20:1455797512547------------->0

Thread-21:1455797512547------------->0

Thread-22:1455797512547------------->0

Thread-23:1455797512547------------->0

Thread-24:1455797512547------------->0

Thread-25:1455797512547------------->0

Thread-26:1455797512547------------->0

Thread-27:1455797512547------------->0

Thread-28:1455797512547------------->0

Thread-29:1455797512547------------->0

Thread-30:1455797512547------------->0

Thread-31:1455797512547------------->0

Thread-32:1455797512547------------->0

Thread-33:1455797512547------------->0

Thread-34:1455797512547------------->0

Thread-35:1455797512547------------->0

Thread-36:1455797512547------------->0

Thread-37:1455797512547------------->0

Thread-38:1455797512562------------->0

Thread-39:1455797512562------------->0

结果是，并发执行的线程数量达到了38个。因为这里一共启动了40个线程，其中38个线程是同一时间(1455797512547)执行的。因此可以确定，并发线程数量和CPU核心数的关系不是一比一的关系了。

另外，JVM到底可以支持多少个线程，google了一下，有了这样一个文章：http://jzhihui.iteye.com/blog/1271122 这里做个记录。