ThreadLocal理解

ThreadLocal

概述

ThreadLocal實例僅作為線程局部變數的==操作類==，以及==線程存儲局部變數時的Key==。真正的線程局部變數是存儲在各自線程的本地，通過Thread類中的ThreadLocal.ThreadLocalMap threadLocals進行存儲。

若希望線上程本地存儲多個局部變數需要使用多個ThreadLocal實例進行操作。

ThreadLocal源碼示例

• set(T v)方法
  
  通過Thread.currentThread()獲取當前的線程實例，然後獲取當前線程實例里的ThreadLocal.ThreadLocalMap threadLocals，若不存在該集合對象則創建。最後，將當前ThreadLocal實例作為Key，將參數值v存儲當前線程的threadLocals集合中。即var3.set(this, var1);

• get()方法
  
  通過Thread.currentThread()獲取當前的線程實例，然後獲取當前線程實例里的ThreadLocal.ThreadLocalMap threadLocals，若該集合對象不為空，則用當前ThreadLocal實例作為Key，從集合對象中獲取之前存儲的值。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源碼示例

Thread類使用ThreadLocal.ThreadLocalMap threadLocals集合對象盛裝線程局部變數。

    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;