ThreadLocal理解

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;