1.类结构及其成员变量
1.1 类结构和注释
类结构代码如下：
代码语言：javascript复制public class ForkJoinWorkerThread extends Thread {
}
ForkJoinWorkerThread继承了Thread类，其注释大意如下：
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ForkJoinWorkerThreadFactory来使用。
1.2 常量
主要有两个：
代码语言：javascript复制final ForkJoinPool pool; // the pool this thread works in
final ForkJoinPool.WorkQueue workQueue; // work-stealing mechanics注释大意： 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InnocuousForkJoinWorkerThread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。
2.构造函数
2.1 ForkJoinWorkerThread(ForkJoinPool pool)
代码语言：javascript复制/**
* Creates a ForkJoinWorkerThread operating in the given pool.
*
* @param pool the pool this thread works in
* @throws NullPointerException if pool is null
*/
protected ForkJoinWorkerThread(ForkJoinPool pool) {
// Use a placeholder until a useful name can be set in registerWorker
super("aForkJoinWorkerThread");
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ForkJoinWorkerThread。 构造函数中最主要的方法就是registerWorker。
2.2 ForkJoinWorkerThread(ForkJoinPool pool, ThreadGroup threadGroup, AccessControlContext acc)
代码语言：javascript复制ForkJoinWorkerThread(ForkJoinPool pool, ThreadGroup threadGroup,
AccessControlContext acc) {
super(threadGroup, null, "aForkJoinWorkerThread");
U.putOrderedObject(this, INHERITEDACCESSCONTROLCONTEXT, acc);
eraseThreadLocals(); // clear before registering
this.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。
代码语言：javascript复制/**
* Erases ThreadLocals by nulling out Thread maps.
*/
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InnocuousForkJoinWorkerThread继承的时候使用。
3.重要的方法
3.1 run
做为Thread，最重要的就是run方法，我们来看看ForkJoinWorkerThread的实现：
代码语言：javascript复制public void run() {
//如果workQueue为空，则抛出异常
if (workQueue.array == null) { // only run once
Throwable exception = null;
try {
//调用onStart方法
onStart();
//调用pool的runWorker方法，运行workQueue
pool.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ForkJoinWorkerThread还支持继承扩展，因此在此定义了两个扩展的方法：
代码语言：javascript复制/**
* Initializes internal state after construction but before
* processing any tasks. If you override this method, you must
* invoke {@code super.onStart()} at the beginning of the method.
* Initialization requires care: Most fields must have legal
* default values, to ensure that attempted accesses from other
* threads work correctly even before this thread starts
* processing tasks.
*/
protected void onStart() {
}
/**
* Performs cleanup associated with termination of this worker
* thread. If you override this method, you must invoke
* {@code super.onTermination} at the end of the overridden method.
*
* @param exception the exception causing this thread to abort due
* to an unrecoverable error, or {@code null} if completed normally
*/
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。
4.内部类InnocuousForkJoinWorkerThread
这个类就是继承了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：
代码语言：javascript复制static final class InnocuousForkJoinWorkerThread extends ForkJoinWorkerThread {
/** The ThreadGroup for all InnocuousForkJoinWorkerThreads */
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)
});
InnocuousForkJoinWorkerThread(ForkJoinPool pool) {
super(pool, innocuousThreadGroup, INNOCUOUS_ACC);
}
@Override // to erase ThreadLocals
void afterTopLevelExec() {
eraseThreadLocals();
}
@Override // to always report system loader
public ClassLoader getContextClassLoader() {
return ClassLoader.getSystemClassLoader();
}
@Override // to silently fail
public void setUncaughtExceptionHandler(UncaughtExceptionHandler x) { }
@Override // paranoically
public void setContextClassLoader(ClassLoader cl) {
throw new SecurityException("setContextClassLoader");
}
/**
* Returns a new group with the system ThreadGroup (the
* topmost, parent-less group) as parent. Uses Unsafe to
* traverse Thread.group and ThreadGroup.parent fields.
*/
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,
"InnocuousForkJoinWorkerThreadGroup");
group = parent;
}
} catch (Exception e) {
throw new Error(e);
}
// fall through if null as cannot-happen safeguard
throw 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。
5. ForkJoinPool中创建工作线程的过程
此时再来结合ForkJoinPool中的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InnocuousForkJoinWorkerThread。
5.1 makeCommonPool创建过程
再ForkJoinPool重的makeCommPool，有如下代码：
代码语言：javascript复制if (factory == null) {
if (System.getSecurityManager() == null)
factory = defaultForkJoinWorkerThreadFactory;
else // use security-managed default
factory = new InnocuousForkJoinWorkerThreadFactory();
}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InnocuousForkJoinWorkerThreadFactory。
5.2 createWorker创建过程
代码语言：javascript复制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ForkJoinWorkerThreadFactory的实现类来创建。
5.3 InnocuousForkJoinWorkerThreadFactory
代码语言：javascript复制static final class InnocuousForkJoinWorkerThreadFactory
implements ForkJoinWorkerThreadFactory {
/**
* An ACC to restrict permissions for the factory itself.
* The constructed workers have no permissions set.
*/
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 ForkJoinWorkerThread newThread(ForkJoinPool pool) {
return (ForkJoinWorkerThread.InnocuousForkJoinWorkerThread)
java.security.AccessController.doPrivileged(
new java.security.PrivilegedAction<ForkJoinWorkerThread>() {
public ForkJoinWorkerThread run() {
return new ForkJoinWorkerThread.
InnocuousForkJoinWorkerThread(pool);
}}, innocuousAcc);
}
}5.4 DefaultForkJoinWorkerThreadFactory
代码语言：javascript复制static final class DefaultForkJoinWorkerThreadFactory
implements ForkJoinWorkerThreadFactory {
public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
return new ForkJoinWorkerThread(pool);
}
}6.总结
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。
