原创

Java中InheritableThreadLocal原理源码分析

如果对ThreadLocal不了解的,请阅读理解ThreadLocal原理分析及内存泄漏问题

ThreadLocal与InheritableThreadLocal都是创建线程私有变量,ThreadLocal绑定的是当前线程,如果希望当前线程的ThreadLocal能够被子线程使用,那么我们需要使用InheritableThreadLocal

简介:

InheritableThreadLocal类的实现非常简单,直接继承了ThreadLocal类,并重写类其中的三个方法。源码如下:

public class InheritableThreadLocal<T> extends ThreadLocal<T> {

    // 该函数在父线程创建子线程,向子线程复制InheritableThreadLocal变量时使用,后面原理时候会提到
    protected T childValue(T parentValue) {
        return parentValue;
    }

     // 从Thread类中的inheritableThreadLocals变量中获取ThreadLocalMap。不再使用threadLocals变量
    ThreadLocalMap getMap(Thread t) {
       return t.inheritableThreadLocals;
    }

    // 赋值的时候给Thread类中的inheritableThreadLocals变量初始化。不再使用threadLocals变量。
    void createMap(Thread t, T firstValue) {
        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
    }
}

原理分析

如此简单,如何实现当前线程的私有变量能够被子线程访问呢?

通过跟踪Thread的构造方法,我们发现只要父线程在构造子线程(调用new Thread())的时候inheritableThreadLocals变量不为空。新生成的子线程会通过ThreadLocal.createInheritedMap方法将父线程inheritableThreadLocals变量有的对象复制到子线程的inheritableThreadLocals变量上。这样就完成了线程间变量的继承与传递

Thread类构造函数最终都会调用init()方法,源码如下:

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;

    // 当前线程为调用new Thread()线程的父线程
    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;
    // 继承父线程的相关属性
    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);

    // 当父线程的inheritableThreadLocals 不为空的时候,会调用 ThreadLocal.createInheritedMap方法
    // 传入的是当前线程(new Thread()线程的父线程)的inheritableThreadLocals变量创建为子线程的inheritableThreadLocals。
    // 这样子线程就拥有了父线程的线程私有变量
    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();
}

使用示例

public class InheritableThreadLocalDemo {

    private static final ThreadLocal<Integer> THREAD_LOCAL = new ThreadLocal<>();

    private static final InheritableThreadLocal<Integer> INHERITABLE_THREAD_LOCAL = new InheritableThreadLocal<>();

    private static class DemoThread extends Thread {
        @Override
        public void run() {
            System.out.println(THREAD_LOCAL.get());
            System.out.println(INHERITABLE_THREAD_LOCAL.get());
        }
    }

    public static void main(String[] args) {
        THREAD_LOCAL.set(123);
        INHERITABLE_THREAD_LOCAL.set(123);
        new DemoThread().start();
        THREAD_LOCAL.remove();
        INHERITABLE_THREAD_LOCAL.remove();
    }
}

Output:

null
123
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