并发编程（五）java Thread.init()源码解读

本文链接：https://blog.youkuaiyun.com/qq_35489012/article/details/128123797

1线程创建

一个新构造的线程对象是由其parent线程来进行空间分配的，而child线程继承了parent是否为Daemon、优先级和加载资源的contextClassLoader以及可继承的ThreadLocal，同时还会分配一个唯一的（sync）ID来标识这个child线程。至此，一个能够运行的线程对象就初始化好了，在堆内存中等待着运行。

下面我们进行源码阅读:

当我们new Thread的时候，会调用init方法，

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

init方法点进去

private void init(ThreadGroup g, Runnable target, String name,
                  long stackSize) {
    init(g, target, name, stackSize, null, true);
}

此时g为null，target为null，name是"Thread-" + nextThreadNum()，stackSize是0，acc是null，inheritThreadLocals是true。

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;

    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();
    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();
}

2源码剖析

接下来逐行分析：

if (name == null) {
    throw new NullPointerException("name cannot be null");
}
this.name = name;

如果当前线程name为null，直接抛出错误，否则继续执行，把传进来的线程名称赋值给当前线程name

Thread parent = currentThread();

currentThread()方法是个native方法，大致意思是那个线程创建的这个线程，父线程就是谁。比如你在main函数里创建了一个线程，那么这个线程的父线程就是main线程。

SecurityManager security = System.getSecurityManager();
if (g == null) {
  
    if (security != null) {
        g = security.getThreadGroup();
    }

    if (g == null) {
        g = parent.getThreadGroup();
    }
}

SecurityManager含义：当运行未知的Java程序的时候，该程序可能有恶意代码（删除系统文件、重启系统等），为了防止运行恶意代码对系统产生影响，需要对运行的代码的权限进行控制，这时候就要启用Java安全管理器。

这里的security主要是为了防止g为null，就是给当前线程找一个线程组，从SecurityManager安全管理器中拿到group。如果此时SecurityManager为null，那就只能从父类里面拿到线group。

尊重线程初始化穿入的threadgroup；次选System security mananger 的 threadgroup；再次选 parent的 threadgroup。

g.checkAccess();

判断当前线程组是不是有权限

if (security != null) {
    if (isCCLOverridden(getClass())) {
        security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
    }
}

判断子类和实现的接口是否有权限

g.addUnstarted();

NEW状态的线程，会添加到当前线程的threadgroup

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();

新的线程的属性（是否为守护线程、优先级）依赖于父类线程。contextClassLoader类加载器也继承父类加载器。继承的inheritedAccessControlContext用于根据其封装的上下文做出系统资源访问决策

this.target = target;
setPriority(priority);

设置当前的Runnable对象和优先级

if (inheritThreadLocals && parent.inheritableThreadLocals != null)
    this.inheritableThreadLocals =
        ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);

这里有一个inheritableThreadLocals，是为了子类的线程也能继承父类线程里的ThreadLocal，所以才有了一个inheritableThreadLocals，就是为了传递。

this.stackSize = stackSize;

tid = nextThreadID();

设置新线程栈大小，可以理解为开辟一块内存空间。 nextThreadID()是分配一个线程tid，并且这个id是synchronize修饰的，保证线程tid的唯一性和线程安全。

private static synchronized long nextThreadID() {
    return ++threadSeqNumber;
}

这章就到这里结束，对线程的创建进行认识