Java源码分析 – Thread线程类源码分析 | Java,源码分析,Thread.线程

1、继承关系：

首先看到Thread类的声明：

class Thread implements Runnable {
/* Make sure registerNatives is the first thing does. */

可知，其实现了Runnable接口，而Runnable接口有一个run()方法，所以Thread也实现了该方法。

2、构造函数：

找到其无参构造函数：

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

这里的第三个参数是设置线程的名称，从下面的代码中可以看出，生成名称的规则是：”Thread-”加上创建的线程的个数（第几个）。

继续查看init方法：

private void init(ThreadGroup g, Runnable target, String name,
long stackSize) {
Thread parent = currentThread();
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();
    }
}
/* 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();

this.group = g;
this.daemon = parent.isDaemon();
this.priority = parent.getPriority();
this.name = name.toCharArray();
if (security == null || isCCLOverridden(parent.getClass()))
    this.contextClassLoader = parent.getContextClassLoader();
else
    this.contextClassLoader = parent.contextClassLoader;
this.inheritedAccessControlContext = AccessController.getContext();
this.target = target;
setPriority(priority);
    if (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();

    this.me = this;
}

初始化时设置了是否为守护线程，优先级，初始化名称。

3、Thread的start方法的实现：

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 || this != me)
throw new IllegalThreadStateException();
group.add(this);
start0();
if (stopBeforeStart) {
stop0(throwableFromStop);
}
}

这里主要的是start0方法；查看其实现：

private native void start0();

这里使用了本地调用，通过C代码初始化线程需要的系统资源。

可见，线程底层的实现是通过C代码去完成的。

4、Thead的run方法的实现：

public void run() {
if (target != null) {
target.run();
}
}

这里的target实际上要保存的是一个Runnable接口的实现的引用：

private Runnable target;

所以使用继承Thread创建线程类时，需要重写run方法，因为默认的run方法什么也不干。

而当我们使用Runnable接口实现线程类时，为了启动线程，需要先勇该线程类实例初始化一个Thread，实际上就执行了如下构造函数：

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

即是把线程类的引用保存到target中。这样，当调用Thread的run方法时，target就不为空了，而是继续调用了target的run方法，所以我们需要实现Runnable的run方法。这样通过Thread的run方法就调用到了Runnable实现类中的run方法。

这也是Runnable接口实现的线程类需要这样启动的原因。