InheritableThreadLocal解析

ThreadLocal是相对于每一个线程自己使用的本地变量，但是在实际的开发中，有这样的一种需求：父线程生成的变量需要传递到子线程中进行使用，那么在使用ThreadLocal似乎就解决不了这个问题，难道这个业务就没办法使用这个本地变量了吗？答案肯定是否定的，ThreadLocal有一个子类InheritableThreadLocal就是为了解决这个问题而产生的，使用这个变量就可以轻松的在子线程中依旧使用父线程中的本地变量。

### 1、InheritableThreadLocal基本的使用

```java
InheritableThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal();
inheritableThreadLocal.set("主线程");
new Thread(() -> {
     System.out.println(Thread.currentThread().getName() + ":" + inheritableThreadLocal.get());
}).start();
System.out.println(Thread.currentThread().getName() + ":" + inheritableThreadLocal.get());
```

输出:

main:主线程
	Thread-0:主线程
	
可以看出已经能从父线程共享数据了

### 2、InheritableThreadLocal源码浅析

```java
public class InheritableThreadLocal<T> extends ThreadLocal<T> {
    /**
     * Computes the child's initial value for this inheritable thread-local
     * variable as a function of the parent's value at the time the child
     * thread is created.  This method is called from within the parent
     * thread before the child is started.
     * <p>
     * This method merely returns its input argument, and should be overridden
     * if a different behavior is desired.
     *
     * @param parentValue the parent thread's value
     * @return the child thread's initial value
     */
    protected T childValue(T parentValue) {
        return parentValue;
    }

/**
     * Get the map associated with a ThreadLocal.
     *
     * @param t the current thread
     */
    ThreadLocalMap getMap(Thread t) {
       return t.inheritableThreadLocals;
    }

/**
     * Create the map associated with a ThreadLocal.
     *
     * @param t the current thread
     * @param firstValue value for the initial entry of the table.
     */
    void createMap(Thread t, T firstValue) {
        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
    }
}
```

可以看出InheritableThreadLocal继承ThreadLocal重载了createMap,getMap,childValue方法。

```java
    // ThreadLocal和InheritableThreadLocal
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
```

Thread中维护了两个ThreadLocalMap,其中inheritableThreadLocals就是可以实现父子线程共享的集合！

```java
    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

/*
     * InheritableThreadLocal values pertaining to this thread. This map is
     * maintained by the InheritableThreadLocal class.
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
```

线程初始化的时候

```java
public Thread(Runnable target) {
    init(null, target, "Thread-" + nextThreadNum(), 0);
}
```