Java中ThreadLocal的使用

public class OuterClass {
    public static String OUTER_DATE = "外部类静态变量加载时间 "+System.currentTimeMillis();
    static {
        System.out.println("外部类静态块加载时间：" + System.currentTimeMillis());
    }
    public OuterClass() {
        System.out.println("外部类构造函数时间：" + System.currentTimeMillis());
    }
    static class InnerStaticClass{
        public static String INNER_STATIC_DATE = "静态内部类静态变量加载时间 "+System.currentTimeMillis();
        private String name;
        static {
            System.out.println("静态内部类静态代码块加载时间：" + System.currentTimeMillis());
        }
    }
    class InnerClass {
        public String INNER_DATE = "";
        public InnerClass() {
            INNER_DATE = "非静态内部类构造器加载时间"+System.currentTimeMillis();
        }
    }
    public static void main(String[] args) {
        OuterClass outer = new OuterClass();
        System.out.println("非静态内部类加载时间: "+outer.new InnerClass().INNER_DATE);
        /**
         * 内部静态类可以直接用，不需要new
         * 静态内部类的加载是代码中需要静态内部类的时候才加载，而不是和外部类一起加载的
         * 加载静态内部类之前，先把外部类的静态变量和静态代码块先执行完
         * 执行完外部类的代码后，再执行静态内部类的 静态变量和静态代码块
         * 静态内部类的 静态变量和静态代码块执行完后，然后执行业务代码
         * new  外部类的时候 。外部类的静态代码块和静态变量先执行，外部类构造函数后执行
         */
        System.out.println("静态内部类加载时间____："+InnerStaticClass.INNER_STATIC_DATE);
    }
}
//什么时候考虑使用静态内部类?
//A类中需要一个B类，但是B类只为A类服务，这种情况不需要将B类单独剥离，只需要在A内部即可
//一个类的构建有非常多参数或者十分复杂的一个对象的时候--引申到了建造者模式

public class ThreadLocaDemo {
    private static ThreadLocal<String> localVar = new ThreadLocal<String>();
    static void print(String str) {
        //打印当前线程中本地内存中本地变量的值
        System.out.println(str + " :" + localVar.get());
        //清除本地内存中的本地变量
        localVar.remove();
    }
    public static void main(String[] args) throws InterruptedException {
        new Thread(new Runnable() {
            public void run() {
                ThreadLocaDemo.localVar.set("local_A");
                print("A");
                //打印本地变量
                System.out.println("after remove : " + localVar.get());
            }
        },"A").start();
        Thread.sleep(1000);
        new Thread(new Runnable() {
            public void run() {
                ThreadLocaDemo.localVar.set("local_B");
                print("B");
                System.out.println("after remove : " + localVar.get());
            }
        },"B").start();
    }
}

 public void set(T value) {
        //1、获取当前线程
        Thread t = Thread.currentThread();
        //2、获取线程中的属性 threadLocalMap ,如果threadLocalMap 不为空，
        //则直接更新要保存的变量值，否则创建threadLocalMap，并赋值
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            // 初始化thradLocalMap 并赋值
            createMap(t, value);
    }

  static class ThreadLocalMap {
        /**
         * The entries in this hash map extend WeakReference, using
         * its main ref field as the key (which is always a
         * ThreadLocal object).  Note that null keys (i.e. entry.get()
         * == null) mean that the key is no longer referenced, so the
         * entry can be expunged from table.  Such entries are referred to
         * as "stale entries" in the code that follows.
         */
        static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;
            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }
    }

//这个是threadlocal 的内部方法
void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }
    //ThreadLocalMap 构造方法
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
            table = new Entry[INITIAL_CAPACITY];
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            table[i] = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }

    public T get() {
        //1、获取当前线程
        Thread t = Thread.currentThread();
        //2、获取当前线程的ThreadLocalMap
        ThreadLocalMap map = getMap(t);
        //3、如果map数据为空，
        if (map != null) {
            //3.1、获取threalLocalMap中存储的值
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        //如果是数据为null，则初始化，初始化的结果，TheralLocalMap中存放key值为threadLocal，值为null
        return setInitialValue();
    }
private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }

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