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ThreadLocal是一个线程局部变量类,他是跟当前线程绑定的,线程与线程之间是相互不影响的。也就是在一个线程中通过ThreadLocal进行set值之后,其他线程get是get不到的,只能自己线程进行get。 但是有时候我们想主线程set值,子线程都能取到,这样的话,ThreadLocal就不适用了。幸好java提供了一个类InheritableThreadLocal,它继承自ThreadLocal, 而且Thread类中也定义了相关的变量,使得这个场景得以实现。
直接上测试代码了:
static ThreadLocalthreadLocal = new ThreadLocal<>(); static InheritableThreadLocal inheritableThreadLocal = new InheritableThreadLocal<>(); public static void main(String[] args) { threadLocal.set("abc"); new Thread(new Runnable() { @Override public void run() { System.out.println(Thread.currentThread().getName()+"====="+threadLocal.get()); } },"thread1").start(); inheritableThreadLocal.set("zhangsan"); new Thread(new Runnable() { @Override public void run() { System.out.println(Thread.currentThread().getName()+"====="+inheritableThreadLocal.get()); } },"thread2").start(); }
通过以上代码可以看出,thread1输出的是null, thread2输出的是zhangsan。
其实现原理也很简单。
InheritableThreadLocal重写了ThreadLocal的三个方法:
childValue,getMap,createMap。
源码如下:
public class InheritableThreadLocalextends 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); }}
当主线程调用 inheritableThreadLocal.set("zhangsan");
之后,调用的实际上是ThreadLocal的set方法,源码如下:
public void set(T value) { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value); }
可以看到set方法会先getMap,get不到就会createMap。主线程第一次调用getMap的时候,主线程对象里面的
inheritableThreadLocals引用默认指向的是null,所以第一次set的时候会执行createMap,因为实例化对象是inheritableThreadLocal对象,所以实际调用的也是他自己的createMap方法。
其实主线程里的set就做了一个事,就是对当前线程里面的inheritableThreadLocal变量进行实例化。
然后主线程里new Thread进行创建子线程的时候,这个时候会执行Thread的init方法,源码如下:
public Thread(Runnable target, String name) { init(null, target, name, 0);}private void init(ThreadGroup g, Runnable target, String name, long stackSize) { init(g, target, name, stackSize, null, true);}private void init(ThreadGroup g, Runnable target, String name, long stackSize, AccessControlContext acc, boolean inheritThreadLocals) { if (name == null) { throw new NullPointerException("name cannot be null"); } this.name = name; Thread parent = currentThread(); SecurityManager security = System.getSecurityManager(); if (g == null) { /* Determine if it's an applet or not */ /* If there is a security manager, ask the security manager what to do. */ if (security != null) { g = security.getThreadGroup(); } /* If the security doesn't have a strong opinion of the matter use the parent thread group. */ if (g == null) { g = parent.getThreadGroup(); } } /* checkAccess regardless of whether or not threadgroup is explicitly passed in. */ g.checkAccess(); /* * Do we have the required permissions? */ 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); if (inheritThreadLocals && parent.inheritableThreadLocals != null) this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals); /* Stash the specified stack size in case the VM cares */ this.stackSize = stackSize; /* Set thread ID */ tid = nextThreadID(); }
在init方法里面有一段代码
这里的parent是针对要创建的子线程而言的,实际是 Thread parent = currentThread(); ,在这里就是主线程本身。
调用了ThreadLocal的createInheritedMap方法,源码如下:
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) { return new ThreadLocalMap(parentMap); }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
ThreadLocalMap是ThreadLocal的内部静态类。在ThreadLocalMap的构造函数中可以看出,这个是private的,其内部有一段代码是 Object value = key.childValue(e.value); 这个key就是ThreadLocal对象,在本示例中,就是inheritableThreadLocal对象,
也就是说调用了调用了inheritableThreadLocal的childValue方法进行获取到值,然后创建entry对象,放到map树里。
至此inheritableThreadLocal重写的三个方法都已经被调用了。
我们可以通过重写InheritableThreadLocal的childValue来改变get方法获取的值。
示例代码如下:
public class TestThreadLocal { static InheritableThreadLocalinheritableThreadLocal1 = new MyThreadLocal<> ("wocaolai"); public static void main(String[] args) { inheritableThreadLocal1.set("zhangsan"); new Thread(new Runnable() { @Override public void run() { System.out.println(Thread.currentThread().getName()+"====="+inheritableThreadLocal1.get()); } },"thread3").start(); } static class MyThreadLocal extends InheritableThreadLocal { public T myvalue; protected T childValue(T parentValue) { return myvalue; } public MyThreadLocal(T myvalue) { this.myvalue = myvalue; } }}
输出的结果是
而不是张三了。
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