ThreadLocal原理解析

涉及到三个实体类

1. ThreadLocal
2. Thread
3. ThreadLocalMap

//ThreadLocalMap是ThreadLocal的内部类
ThreadLocalMap{
	//类似于HashMap，键值分别是ThreadLocal、对应的值
	Entry<ThreadLocal, Object>[] entry;
}

Thread{
	//保存了线程私有的ThreadLocalMap
	ThreadLocalMap map;
}

ThreadLocal{
	//获取被哪个线程调用的
	Thread t = Thread.currentThread();
	//获取当前线程在该ThreadLocal下的
	get(){
		//获取该线程的ThreadLocalMap
		map_temp = t.map;
		//如果map_temp为空，则根据自己重写的初始化策略进行初始化，并存入对应的ThreadLocalMap中对应的槽中
		if(map_temp == null){
			initVal = init();
			map_temp.set(this, initVal);
		}
		//类似于HashMap根据键查找值，这里的键是ThreadLocal对象本身
		return map_temp.get(this);
	}
}

可继承的InheritableThreadLocal

public class Test {

    private static final ThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal<>();

    public static void main(String[] args) {
		//父线程
        new Thread(()->{
            inheritableThreadLocal.set("hzw");
			//子线程
            new Thread(()->{
                System.out.println(inheritableThreadLocal.get());
            }).start();

        }).start();
    }
}

运行结果是打印出hzw
准确来说是复制父线程的数据，当父线程remove掉后，子线程还可以获取到，示例如下

public class Test {

    private static final ThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal<>();

    public static void main(String[] args) {

        new Thread(()->{
            inheritableThreadLocal.set("hzw");

            new Thread(()->{
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(inheritableThreadLocal.get());
            }).start();

            inheritableThreadLocal.remove();
            System.out.println("属性移除");
        }).start();
    }
}

运行结果是hzw，说明父线程移除后，子线程仍保留

源码解析：4个主要阶段

public class Test {

    private static final ThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal<>();

    public static void main(String[] args) {
		//父线程
        new Thread(()->{   ..........................................1
            inheritableThreadLocal.set("hzw");   ....................2
			//子线程
            new Thread(()->{   ......................................3
                System.out.println(inheritableThreadLocal.get());   .4
            }).start();

        }).start();
    }
}

1.创建父线程(第1行对应的代码)

 if (inheritThreadLocals && parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);

上述代码是Thread初始化时执行的，由于父线程的父线程是main，main中还没进行ThreadLocal操作，所以parent.inheritableThreadLocals==null，上述if逻辑内代码不执行

2.父线程存储值(第2行对应的代码)

InheritableThreadLocal没有重写set方法，所以set方法在ThreadLocal中调用，紧接着set方法中调用了createMap方法，此方法被重写，所以执行以下代码：

    void createMap(Thread t, T firstValue) {
        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
    }

父子类createMap的唯一区别：ThreadLocal中创建的Map存储在t.threadLocals中，而InheritableThreadLocal中的createMap方法存储在t.inheritableThreadLocals中。

3.子线程初始化(第3行对应的代码)

 if (inheritThreadLocals && parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);

第二步后，父线程的inheritableThreadLocals不为空，上述代码的if逻辑会执行，传入父线程的inheritedMap，为当前线程即子线程创建InheritedMap，并赋值给inheritableThreadLocals。具体创建过程如下：

		private ThreadLocalMap(ThreadLocalMap parentMap) {
            Entry[] parentTable = parentMap.table;
            int len = parentTable.length;
            setThreshold(len);
            table = new Entry[len];

            for (int j = 0; j < len; j++) {
                Entry e = parentTable[j];
                if (e != null) {
                    @SuppressWarnings("unchecked")
                    ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
                    if (key != null) {
                        Object value = key.childValue(e.value);
                        Entry c = new Entry(key, value);
                        int h = key.threadLocalHashCode & (len - 1);
                        while (table[h] != null)
                            h = nextIndex(h, len);
                        table[h] = c;
                        size++;
                    }
                }
            }
        }

其实就是将父线程的map复制给子线程，所以父线程map移除后，子线程会保留

4.子线程取值(第4行对应的代码)

 public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);   .......................1
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }

调用的是ThreadLocal中的get，我们主要关心ThreadLocalMap map = getMap(t);这一行。这个和createMap一样被重写了，子类和父类实现逻辑是分别返回threadLocals和interitableThreadLocals，相当于set和get方法。