java Thread源码分析

一、使用 java 多线程

java多线程其中两种使用方式：

1、继承 Thread 类

2、实现 Runnable 接口

public class ThreadTest {
    public static void main(String[] args) {
        Thread t1 = new MyThread("thread-0001");
        t1.start();
        MyRunnable mr = new MyRunnable();
        Thread t2 = new Thread(mr, "thread-0002");
        t2.start();
    }
    private static class MyThread extends Thread{
        public MyThread(String name) {
            this.setName(name);
        }
        @Override
        public void run() {
            System.out.println(this.getName()+" thread run....");
        }
    }
    private static class MyRunnable implements Runnable{
        @Override
        public void run() {
            System.out.println(Thread.currentThread().getName()+" runable run....");    
        }
    }
}

二、线程初始化

　　继承 Thread 和 实现 Runnable 的方式都要经过初始化Thread构造函数的方式设置相关参数的过程。

　　构造函数如下：

public Thread() {
    init(null, null, "Thread-" + nextThreadNum(), 0);
}

public Thread(Runnable target) {
    init(null, target, "Thread-" + nextThreadNum(), 0);
}

public Thread(ThreadGroup group, Runnable target) {
    init(group, target, "Thread-" + nextThreadNum(), 0);
}

public Thread(String name) {
    init(null, null, name, 0);
}

public Thread(ThreadGroup group, String name) {
    init(group, null, name, 0);
}

public Thread(Runnable target, String name) {
    init(null, target, name, 0);
}

public Thread(ThreadGroup group, Runnable target, String name) {
    init(group, target, name, 0);
}

public Thread(ThreadGroup group, Runnable target, String name, long stackSize) {
    init(group, target, name, stackSize);
}

　　上述构造函数涉及的参数：

　　ThreadGroup group（该线程所属线程组）、Runnable targer、String name（线程名）、long stackSize（线程堆栈大小，不知道有什么作用，日后再填坑）。

　　最终会执行 init 函数：

private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc,
                      boolean inheritThreadLocals) {
        if (name == null) {
            throw new NullPointerException("name cannot be null");
        }
        
        // 线程名
        this.name = name;

        // 创建该线程时的线程（即父线程）
        Thread parent = currentThread();
        SecurityManager security = System.getSecurityManager();
        // 线程组
        if (g == null) {
            /* Determine if it's an applet or not */

            /* If there is a security manager, ask the security manager
               what to do. */
            if (security != null) {
                g = security.getThreadGroup();
            }

            /* If the security doesn't have a strong opinion of the matter
               use the parent thread group. */
            // 没有设置线程组时，默认线程组为父线程的线程组
            if (g == null) {
                g = parent.getThreadGroup();
            }
        }

        /* checkAccess regardless of whether or not threadgroup is
           explicitly passed in. */
        g.checkAccess();

        /*
         * Do we have the required permissions?
         */
        if (security != null) {
            if (isCCLOverridden(getClass())) {
                security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
            }
        }

        g.addUnstarted();

        // 设置线程组
        this.group = g;
        // 设置
        this.daemon = parent.isDaemon();
        // 设置优先级和父线程的优先级相同
        this.priority = parent.getPriority();
        if (security == null || isCCLOverridden(parent.getClass()))
            this.contextClassLoader = parent.getContextClassLoader();
        else
            this.contextClassLoader = parent.contextClassLoader;
        this.inheritedAccessControlContext =
                acc != null ? acc : AccessController.getContext();
        // Runnable target
        this.target = target;
        setPriority(priority);
        if (inheritThreadLocals && parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;

        /* Set thread ID */
        // 设置线程的ID（threadSeqNumber），同步递增，每个线程都会生成一个thread ID
        // 另外一个同步递增的ID (threadInitNumber)，这个在没有显式指定线程名的时候，默认生成线程名（Thread-N）。
        // 所以，threadSeqNumber >= threadInitNumber
        tid = nextThreadID();
    }

　　t1.start()会调用本地方法start0()启动线程：

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. */
        // 将线程添加到线程组中
         group.add(this);

        boolean started = false;
        try {
            // navite修饰的方法
            start0();
            started = true;
        } finally {
            try {
                if (!started) {
                    // 线程启动失败，从线程组中移除线程
                    group.threadStartFailed(this);
                }
            } catch (Throwable ignore) {
                /* do nothing. If start0 threw a Throwable then
                  it will be passed up the call stack */
            }
        }
    }