推薦：​​Java并發程式設計彙總​​

啟動線程的正确姿勢

通過之前的分析，我們知道了有兩種定義線程執行邏輯的方法，而建立線程隻有一種方法，也就是​

​new​

​​一個​

​Thread​

​​類的執行個體。

​JAVA實作多線程到底有多少種方法？​​

我們如何啟動線程呢？其實很簡單，調用​

​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);
        // 設定線程名稱
        thread.setName("thread_kaven");
        thread.start();
    }
}      

輸出：

thread_kaven      

很顯然，​

​start()​

​​啟動了線程，并且是在一個新的線程中去執行任務的，但是為什麼一定要調用​

​start()​

​​，而不直接調用該線程的執行邏輯​

​run()​

​​呢？

我們來測試調用​​

​run()​

​的情況。

package startThread;

public class test {
    public static void main(String[] args) {
        Runnable runnable = ()->{
            System.out.println(Thread.currentThread().getName());
        };

        Thread thread = new Thread(runnable);
        // 設定線程名稱
        thread.setName("thread_kaven");
        thread.run();
    }
}      

輸出：

main      

很顯然，調用​

​run()​

​​，并沒有建立新的線程去執行任務，​

​run()​

​​還是在​

​main​

​線程中執行的。

我們可以得到一個簡單的結論：

​

​run()​

​​隻是一個類中的普通方法，調用​

​run()​

​​跟調用普通方法一樣。

而調用​​

​start()​

​​，則會做一系列工作去建立新線程，然後在新線程中執行​

​run()​

​裡面的任務内容。

我們可以多次調用​

​start()​

​嗎？我們來測試一下。

package startThread;

public class test {
    public static void main(String[] args) {
        Runnable runnable = ()->{
            System.out.println(Thread.currentThread().getName());
        };

        Thread thread = new Thread(runnable);
        // 設定線程名稱
        thread.setName("thread_kaven");
        thread.start();
        thread.start();
    }
}      

輸出：

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

/**
     * 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 */
            }
        }
    }