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ForkJoinWorker>ForkJoinWorkerThread实际上非常简单，就是结合ForkJoinPool，然后根据其需要，创建合适的线程的过程。这里面值得我们借鉴的是，如果需要创建无其他访问权限的线程，实际上这两种线程大部分内容都是相同的，因此可以通过继承来复用大部分代码。之后定义两个factory，让最终的用户根据需要选择factory。

一、类结构及其成员变量

1.1 类结构和注释

类结构代码如下：

public class ForkJoinWorker>ForkJoinWorkerThread extends Thread {}

ForkJoinWorker>ForkJoinWorkerThread继承了Thread类，ForkJoinWorker>ForkJoinWorkerThread是由ForkJoinPool管理的线程，该线程执行ForkJoinTask。此类仅可做为扩展功能的需要而被集成，因为没有提供可以调度或者可重新的方法。但是，你可以覆盖主任务处理循环周围初始化和终止方法。如果确实创建了这个类的子类，还需要在ForkJoinPool中提供自定义的ForkJoinWorker>ForkJoinWorkerThreadFactory来使用。

1.2 常量

主要有两个：

final ForkJoinPool pool;                // the pool this thread works in
final ForkJoinPool.WorkQueue workQueue; // work-stealing mechanics

ForkJoinWorker>ForkJoinWorkerThreads由ForkJoinPools管理，并执行ForkJoinTasks。请参见ForkJoinPool的内部文档。此类仅仅维护了指向pool和WorkQueue的链接。pool字段在构造的时候直接设置。但是直到对registerWorker调用完成之后，才设置workQueue字段。这将导致可见性竞争，可以通过要求workQueue字段仅由其所属线程访问来规避这个问题。对于InnocuousForkJoinWorker>ForkJoinWorkerThread子类的支持，要求我们在此处和子类中破坏很多封装，通过Unsafe以访问和设置Thread字段。

这是两个final修饰的常量，智能初始化一次。

二、构造函数

2.1 ForkJoinWorker>ForkJoinWorkerThread(ForkJoinPool pool)

protected ForkJoinWorker>ForkJoinWorkerThread(ForkJoinPool pool) {// Use a placeholder until a useful name can be set in registerWorkersuper("aForkJoinWorker>ForkJoinWorkerThread");this.pool = pool;this.workQueue = pool.registerWorker(this);
}

其构造函数主要是创建一个在给定pool中的ForkJoinWorker>ForkJoinWorkerThread。构造函数中最主要的方法就是registerWorker。

2.2 ForkJoinWorker>ForkJoinWorkerThread(ForkJoinPool pool, ThreadGroup threadGroup, AccessControlContext acc)

ForkJoinWorker>ForkJoinWorkerThread(ForkJoinPool pool, ThreadGroup threadGroup,AccessControlContext acc) {super(threadGroup, null, "aForkJoinWorker>ForkJoinWorkerThread");U.putOrderedObject(this, INHERITEDACCESSCONTROLCONTEXT, acc);eraseThreadLocals(); // clear before registeringthis.pool = pool;this.workQueue = pool.registerWorker(this);
}

这个方法需要注意的是，采用unSafe方法，在这个类的INHERITEDACCESSCONTROLCONTEXT位置处，设置传入的AccessControlContext对象。
之后调用方法eraseThreadLocals将threadLocals清除。
之后与同用的构造函数一致。
擦除ThreadLocal也是采用UnSafe来完成。通过putObject将ThreadLocals的位置设置为null。

final void eraseThreadLocals() {U.putObject(this, THREADLOCALS, null);U.putObject(this, INHERITABLETHREADLOCALS, null);
}

实际上这个方法将会提供给InnocuousForkJoinWorker>ForkJoinWorkerThread继承的时候使用。

三、核心方法

3.1 run

做为Thread，最重要的就是run方法，我们来看看ForkJoinWorker>ForkJoinWorkerThread的实现：

public void run() {//如果workQueue为空，则抛出异常if (workQueue.array == null) { // only run onceThrowable exception = null;try {//调用onStart方法onStart();//调用pool的runWorker方法，运行workQueuepool.runWorker(workQueue);} catch (Throwable ex) {exception = ex;} finally {//操作完之后处理try {//调用onTermination方法onTermination(exception);} catch (Throwable ex) {//如果出现异常，则进行异常处理if (exception == null)exception = ex;} finally {//最终需要指向deregister方法pool.deregisterWorker(this, exception);}}}
}

runWorker方法实际上是对workQueue结合随机魔数，选择一个workQueue进行遍历，调用scan方法，如果不为空则执行，反之则wait。
其中registerWorker与deregisterWorker方法，我们可以参考前面的ForkJoinPool源码解读。

3.2 定义的可扩展方法

由于ForkJoinWorker>ForkJoinWorkerThread还支持继承扩展，因此在此定义了两个扩展的方法：

protected void onStart() {
}protected void onTermination(Throwable exception) {
}

这两个方法用于执行之前和之后，onStart用于run实际执行之前，执行一些初始化操作。onTermination用于run实际执行之后，执行一些清理操作。

四、内部类InnocuousForkJoinWorker>ForkJoinWorkerThread

这个类就是继承了ForkJoinWorker>ForkJoinWorkerThread的一个实现类。此类定义了一个没有任何权限、也非用户定义的任何线程组的线程。这个线程在运行完每个top的task之后，会擦除所有的ThreadLocals。

源码如下：

static final class InnocuousForkJoinWorker>ForkJoinWorkerThread extends ForkJoinWorker>ForkJoinWorkerThread {/** The ThreadGroup for all InnocuousForkJoinWorker>ForkJoinWorkerThreads */private static final ThreadGroup innocuousThreadGroup =createThreadGroup();/** An AccessControlContext supporting no privileges */private static final AccessControlContext INNOCUOUS_ACC =new AccessControlContext(new ProtectionDomain[] {new ProtectionDomain(null, null)});InnocuousForkJoinWorker>ForkJoinWorkerThread(ForkJoinPool pool) {super(pool, innocuousThreadGroup, INNOCUOUS_ACC);}@Override // to erase ThreadLocalsvoid afterTopLevelExec() {eraseThreadLocals();}@Override // to always report system loaderpublic ClassLoader getContextClassLoader() {return ClassLoader.getSystemClassLoader();}@Override // to silently failpublic void setUncaughtExceptionHandler(UncaughtExceptionHandler x) { }@Override // paranoicallypublic void setContextClassLoader(ClassLoader cl) {throw new SecurityException("setContextClassLoader");}private static ThreadGroup createThreadGroup() {try {sun.misc.Unsafe u = sun.misc.Unsafe.getUnsafe();Class<?> tk = Thread.class;Class<?> gk = ThreadGroup.class;long tg = u.objectFieldOffset(tk.getDeclaredField("group"));long gp = u.objectFieldOffset(gk.getDeclaredField("parent"));ThreadGroup group = (ThreadGroup)u.getObject(Thread.currentThread(), tg);while (group != null) {ThreadGroup parent = (ThreadGroup)u.getObject(group, gp);if (parent == null)return new ThreadGroup(group,"InnocuousForkJoinWorker>ForkJoinWorkerThreadGroup");group = parent;}} catch (Exception e) {throw new Error(e);}// fall through if null as cannot-happen safeguardthrow new Error("Cannot create ThreadGroup");}
}

AccessControlContext INNOCUOUS_ACC 定义了一个不支持任何特权访问的AccessControlContext。这个类会创建一个单独的threadGroup，以确保其不属于任何一个用户创建的ThreadGroup。

五、ForkJoinPool中创建工作线程的过程

此时再来结合ForkJoinPool中的ForkJoinWorker>ForkJoinWorkerThreadFactory，就能明白ForkJoinThread的创建意义了。ForkJoinPool根据访问权限的需要，定义了采用默认的创建方法，还是创建InnocuousForkJoinWorker>ForkJoinWorkerThread。

5.1 makeCommonPool创建过程

再ForkJoinPool重的makeCommPool，有如下代码：

if (factory == null) {if (System.getSecurityManager() == null)factory = defaultForkJoinWorker>ForkJoinWorkerThreadFactory;else // use security-managed defaultfactory = new InnocuousForkJoinWorker>ForkJoinWorkerThreadFactory();
}

这里也就是说，如果System.getSecurityManager()为null，则返回默认的ThreadFactory,而不为null，则说，使用了默认的安全管理级别，因此将创建InnocuousForkJoinWorker>ForkJoinWorkerThreadFactory。

5.2 createWorker创建过程

ForkJoinWorker>ForkJoinWorkerThreadFactory fac = factory;
try {if (fac != null && (wt = fac.newThread(this)) != null) {wt.start();return true;}
} catch (Throwable rex) {ex = rex;
}

也就是说，createWorker根据ForkJoinWorker>ForkJoinWorkerThreadFactory的实现类来创建。

5.3 InnocuousForkJoinWorker>ForkJoinWorkerThreadFactory

static final class InnocuousForkJoinWorker>ForkJoinWorkerThreadFactoryimplements ForkJoinWorker>ForkJoinWorkerThreadFactory {private static final AccessControlContext innocuousAcc;static {Permissions innocuousPerms = new Permissions();innocuousPerms.add(modifyThreadPermission);innocuousPerms.add(new RuntimePermission("enableContextClassLoaderOverride"));innocuousPerms.add(new RuntimePermission("modifyThreadGroup"));innocuousAcc = new AccessControlContext(new ProtectionDomain[] {new ProtectionDomain(null, innocuousPerms)});}public final ForkJoinWorker>ForkJoinWorkerThread newThread(ForkJoinPool pool) {return (ForkJoinWorker>ForkJoinWorkerThread.InnocuousForkJoinWorker>ForkJoinWorkerThread)java.security.AccessController.doPrivileged(new java.security.PrivilegedAction<ForkJoinWorker>ForkJoinWorkerThread>() {public ForkJoinWorker>ForkJoinWorkerThread run() {return new ForkJoinWorker>ForkJoinWorkerThread.InnocuousForkJoinWorker>ForkJoinWorkerThread(pool);}}, innocuousAcc);}
}

5.4 DefaultForkJoinWorker>ForkJoinWorkerThreadFactory

static final class DefaultForkJoinWorker>ForkJoinWorkerThreadFactoryimplements ForkJoinWorker>ForkJoinWorkerThreadFactory {public final ForkJoinWorker>ForkJoinWorkerThread newThread(ForkJoinPool pool) {return new ForkJoinWorker>ForkJoinWorkerThread(pool);}
}