ThreadLocal理解

Posted 2021-01-03 huangyo

ThreadLocal

概述

ThreadLocal实例仅作为线程局部变量的==操作类==，以及==线程存储局部变量时的Key==。真正的线程局部变量是存储在各自线程的本地，通过Thread类中的ThreadLocal.ThreadLocalMap threadLocals进行存储。

若希望在线程本地存储多个局部变量需要使用多个ThreadLocal实例进行操作。

ThreadLocal源码示例

• set(T v)方法
  
  通过Thread.currentThread()获取当前的线程实例，然后获取当前线程实例里的ThreadLocal.ThreadLocalMap threadLocals，若不存在该集合对象则创建。最后，将当前ThreadLocal实例作为Key，将参数值v存储当前线程的threadLocals集合中。即var3.set(this, var1);

• get()方法
  
  通过Thread.currentThread()获取当前的线程实例，然后获取当前线程实例里的ThreadLocal.ThreadLocalMap threadLocals，若该集合对象不为空，则用当前ThreadLocal实例作为Key，从集合对象中获取之前存储的值。即var3 = var2.getEntry(this);

    public T get() {
        Thread var1 = Thread.currentThread();
        ThreadLocal.ThreadLocalMap var2 = this.getMap(var1);
        if (var2 != null) {
            ThreadLocal.ThreadLocalMap.Entry var3 = var2.getEntry(this);
            if (var3 != null) {
                Object var4 = var3.value;
                return var4;
            }
        }

        return this.setInitialValue();
    }

    private T setInitialValue() {
        Object var1 = this.initialValue();
        Thread var2 = Thread.currentThread();
        ThreadLocal.ThreadLocalMap var3 = this.getMap(var2);
        if (var3 != null) {
            var3.set(this, var1);
        } else {
            this.createMap(var2, var1);
        }

        return var1;
    }

    public void set(T var1) {
        Thread var2 = Thread.currentThread();
        ThreadLocal.ThreadLocalMap var3 = this.getMap(var2);
        if (var3 != null) {
            var3.set(this, var1);
        } else {
            this.createMap(var2, var1);
        }

    }

    public void remove() {
        ThreadLocal.ThreadLocalMap var1 = this.getMap(Thread.currentThread());
        if (var1 != null) {
            var1.remove(this);
        }

    }

    ThreadLocal.ThreadLocalMap getMap(Thread var1) {
        return var1.threadLocals;
    }

    void createMap(Thread var1, T var2) {
        var1.threadLocals = new ThreadLocal.ThreadLocalMap(this, var2);
    }

Thread源码示例

Thread类使用ThreadLocal.ThreadLocalMap threadLocals集合对象盛装线程局部变量。

    public class Thread implements Runnable {
    /* Make sure registerNatives is the first thing <clinit> does. */
    private static native void registerNatives();
    static {
        registerNatives();
    }
    private volatile String name;
    private int            priority;
    private Thread         threadQ;
    private long           eetop;
    /* Whether or not to single_step this thread. */
    private boolean     single_step;
    /* Whether or not the thread is a daemon thread. */
    private boolean     daemon = false;
    /* JVM state */
    private boolean     stillborn = false;
    /* What will be run. */
    private Runnable target;
    /* The group of this thread */
    private ThreadGroup group;
    /* The context ClassLoader for this thread */
    private ClassLoader contextClassLoader;
    /* The inherited AccessControlContext of this thread */
    private AccessControlContext inheritedAccessControlContext;
    /* For autonumbering anonymous threads. */
    private static int threadInitNumber;
    private static synchronized int nextThreadNum() {
        return threadInitNumber++;
    }
    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;