启动线程的正确姿势

悠悠 2023-07-05 11:45 18阅读 0赞

推荐：[Java并发编程汇总][Java]

## 启动线程的正确姿势 ##

通过之前的分析，我们知道了有两种定义线程执行逻辑的方法，而创建线程只有一种方法，也就是`new`一个`Thread`类的实例。  
[JAVA实现多线程到底有多少种方法？][JAVA]

我们如何启动线程呢？其实很简单，调用`start()`即可。

package startThread;
    
    public class test { 
        public static void main(String[] args) { 
            Runnable runnable = ()->{ 
                System.out.println(Thread.currentThread().getName());
            };
    
            Thread thread = new Thread(runnable);
            // 设置线程名称
            thread.setName("thread_kaven");
            thread.start();
        }
    }

输出：

thread_kaven

很显然，`start()`启动了线程，并且是在一个新的线程中去执行任务的，但是为什么一定要调用`start()`，而不直接调用该线程的执行逻辑`run()`呢？  
我们来测试调用`run()`的情况。

输出：

main

很显然，调用`run()`，并没有创建新的线程去执行任务，`run()`还是在`main`线程中执行的。

我们可以得到一个简单的结论：

`run()`只是一个类中的普通方法，调用`run()`跟调用普通方法一样。  
而调用`start()`，则会做一系列工作去创建新线程，然后在新线程中执行`run()`里面的任务内容。

我们可以多次调用`start()`吗？我们来测试一下。

输出：

thread_kaven
    Exception in thread "main" java.lang.IllegalThreadStateException
    	at java.lang.Thread.start(Thread.java:708)
    	at startThread.test.main(test.java:13)

报错了，为什么不能多次调用`start()`呢？我们来看一下`start()`源码。

/** * Causes this thread to begin execution; the Java Virtual Machine * calls the <code>run</code> method of this thread. * <p> * The result is that two threads are running concurrently: the * current thread (which returns from the call to the * <code>start</code> method) and the other thread (which executes its * <code>run</code> method). * <p> * It is never legal to start a thread more than once. * In particular, a thread may not be restarted once it has completed * execution. * * @exception IllegalThreadStateException if the thread was already * started. * @see #run() * @see #stop() */
        public synchronized void start() { 
            /** * This method is not invoked for the main method thread or "system" * group threads created/set up by the VM. Any new functionality added * to this method in the future may have to also be added to the VM. * * A zero status value corresponds to state "NEW". */
            if (threadStatus != 0)
                throw new IllegalThreadStateException();
    
            /* Notify the group that this thread is about to be started * so that it can be added to the group's list of threads * and the group's unstarted count can be decremented. */
            group.add(this);
    
            boolean started = false;
            try { 
                start0();
                started = true;
            } finally { 
                try { 
                    if (!started) { 
                        group.threadStartFailed(this);
                    }
                } catch (Throwable ignore) { 
                    /* do nothing. If start0 threw a Throwable then it will be passed up the call stack */
                }
            }
        }

从源码可以知道，调用`start()`时，会判断`threadStatus != 0`，很明显，刚刚的异常就是这个地方抛出来的，通过源码可以知道`threadStatus`是记录线程的状态，初始默认为`0`，当线程启动后，该线程的状态就改变了，所以`threadStatus`的值也就改变了，当第二次调用`start()`时，就会抛出异常。

[Java]: https://kaven.blog.csdn.net/article/details/104233588
[JAVA]: https://blog.csdn.net/qq_37960603/article/details/103922663