InheritableThreadLocal使用详解
执行结果如下：
value:mainThread
value:mainThread
InheritableThreadLocal原理分析[#](()
先看下InheritableThreadLocal的源代码:
public class InheritableThreadLocal extends ThreadLocal {
protected T childValue(T parentValue) {
return parentValue;
}
ThreadLocalMap getMap(Thread t) {
return t.inheritableThreadLocals;
}
void createMap(Thread t, T firstValue) {
t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
}
}
这个类继承了ThreadLocal，并且重写了getMap和createMap方法，区别就是将 ThreadLocal 中的 threadLocals 换成了 inheritableThreadLocals，这两个变量都是ThreadLocalMap类型，并且都是Thread类的属性。
下面就一步步来看下InheritableThreadLocal为什么能拿到父线程中的ThreadLocal值。
step1：InheritableThreadLocal获取值先调用了get方法，所以我们直接看看get方法都做了些啥。
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings(“unchecked”)
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
从上面的代码可以看出，get方法和ThreadLocal中是一样的，唯一有区别的就是其中的getMap方法重写了，返回的是inheritableThreadLocals属性。这个属性也是一个ThreadLocalMap类型的变量。那么从这边就可以推断出来：肯定是在某处将父线程中的ThreadLocal值赋值到了子线程的inheritableThreadLocals中。
step2：在源代码中搜索哪些地方使用到了inheritableThreadLocals这个属性，最后找到这段代码:
private void init(ThreadGroup g, Runnable target, String name,long stackSize, AccessControlContext acc) {
if (name == null) {
throw new NullPointerException(“name cannot be null”);
}
this.name = name.toCharArray();
Thread parent = currentThread();
SecurityManager security = System.getSecurityManager();
if (g == null) {
if (security != null) {
g = security.getThreadGroup();
}
if (g == null) {
g = parent.getThreadGroup();
}
}
g.checkAccess();
if (security != null) {
if (isCCLOverridden(getClass())) {
security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
}
}
g.addUnstarted();
this.group = g;
this.daemon = parent.isDaemon();
this.priority = parent.getPriority();
if (security == null || isCCLOverridden(parent.getClass()))
this.contextClassLoader = parent.getContextClassLoader();
else
this.contextClassLoader = parent.contextClassLoader;
this.inheritedAccessControlContext =
acc != null ? acc : AccessController.getContext();
this.target = target;
setPriority(priority);
//1. 这边先判断了父线程中inheritableThreadLocals属性是否为空，不为空的话就复制给子线程
if (parent.inheritableThreadLocals != null)
this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
/* Stash the specified stack size in case the VM cares */