创建Thread实例流程

执行静态代码块中registerNatives()方法，底层线程注册方法。

public
class Thread implements Runnable {
    /* Make sure registerNatives is the first thing <clinit> does. */
    private static native void registerNatives();
    static {
        registerNatives();
    }
    ...

init方法，初始化线程环境。无论通过哪种方式创建Thread都会调用init方法。

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

2.1 设置线程名称

/**
 * Initializes a Thread.
 *
 * @param g the Thread group
 * @param target the object whose run() method gets called
 * @param name the name of the new Thread
 * @param stackSize the desired stack size for the new thread, or
 *        zero to indicate that this parameter is to be ignored.
 * @param acc the AccessControlContext to inherit, or
 *            AccessController.getContext() if null
 * @param inheritThreadLocals if {@code true}, inherit initial values for
 *            inheritable thread-locals from the constructing thread
 */
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");
    }

    this.name = name;

2.2 设置新线程父线程为当前线程

    Thread parent = currentThread();

2.3 获取系统安全管理SecurityManager，并获得线程组

    SecurityManager security = System.getSecurityManager();
    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. */
        if (g == null) {
            g = parent.getThreadGroup();
        }
    }

2.4 获取线程组的权限检查

    /* 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);
        }
    }

2.5 在线程组增加未启动的线程数量

    g.addUnstarted();

2.6 设置新线程的属性：守护线程属性、优先级、堆栈大小、线程组、线程安全控制上下文等。

    this.group = g;
    this.daemon = parent.isDaemon();
    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();
    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 */
    tid = nextThreadID();
}

start方法

3.1 线程组中减少未启动线程数量

/**
 * 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);

nUnstartedThreads--;

void add(Thread t) {
    synchronized (this) {
        if (destroyed) {
            throw new IllegalThreadStateException();
        }
        if (threads == null) {
            threads = new Thread[4];
        } else if (nthreads == threads.length) {
            threads = Arrays.copyOf(threads, nthreads * 2);
        }
        threads[nthreads] = t;

        // This is done last so it doesn't matter in case the
        // thread is killed
        nthreads++;

        // The thread is now a fully fledged member of the group, even
        // though it may, or may not, have been started yet. It will prevent
        // the group from being destroyed so the unstarted Threads count is
        // decremented.
        nUnstartedThreads--;
    }
}

3.2 调用本地方法start启动线程，底层native方法调用run执行

    boolean started = false;
    try {
        start0();
        started = true;
    }

3.3 启动失败，从线程组中移除该线程，并且增加未启动线程数量。

    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 */
        }
    }
}

Thread源码分析

创建Thread实例流程