android 进程/线程管理（二）----关于线程的迷思
一：进程和线程的由来
进程是计算机科技发展的过程的产物。
最早计算机发明出来，是为了解决数学计算而发明的。每解决一个问题，就要打纸带，也就是打点。
后来人们发现可以批量的设置命令，由计算机读取这些命令，并挨个执行。
在使用的过程中，有一个问题，如果要做I/O操作，是非常耗时的，这个时候CPU是闲着的，这对于计算机资源是一个巨大的浪费。
于是，人们发明了进程这个东西。每个程序就是一个进程，由操作系统管理，当进行复杂的耗时操作是，CPU可以调度处理其他的进程，从而是性能在整体上提高。
线程的目的：
当CPU调度的某个进程时，该进程正在做网络操作，这个时候，如果用户点击某个按钮，是无法及时响应的，体验非常不好。于是，比进程更“小”的调度单位出现了----线程。
我把与用于响应的操作放在一个线程，把耗时的操作放在其他线程，这个用户可以看到界面快速的响应，没有延时的效果。
这也是anroid等现在主流操作系统的编程范式：
把UI操作绑定的主线程，由工作线程处理其他的任务。主要是耗时的任务。
二：线程的启动过程
创建和启动一个新线程，无论经过多少层的封装，最终的目的就是由操作系统提供的api来完成。
以下就是从java thread出发，层层分析，一直到linux的pthread结束。
Thread源码位于：
libcore/libdvm/src/main/java/java/lang/Thread.java
public class Thread implements Runnable { 可见thread是实现了一个runnable接口。
public interface Runnable { /** * Starts executing the active part of the class' code. This method is * called when a thread is started that has been created with a class which * implements {@code Runnable}. */ public void run(); } runnable什么也没有，就是run函数。
所以线程的根本就是 创建一个新的线程，运行run方法。
下面我们看看创建线程的过程。
public Thread() { create(null, null, null, 0); } public Thread(Runnable runnable) { create(null, runnable, null, 0); } 如上所示，常见的应用中使用thread的方法就是
a.定义一个新thread子类，实现run方法。
b.直接传递run给thread作为参数。
接下去我们看下create方法：
private void create(ThreadGroup group, Runnable runnable, String threadName, long stackSize) { Thread currentThread = Thread.currentThread(); if (group == null) { group = currentThread.getThreadGroup(); } if (group.isDestroyed()) { throw new IllegalThreadStateException("Group already destroyed"); } this.group = group; synchronized (Thread.class) { id = ++Thread.count; } if (threadName == null) { this.name = "Thread-" + id; } else { this.name = threadName; } this.target = runnable; this.stackSize = stackSize; this.priority = currentThread.getPriority(); this.contextClassLoader = currentThread.contextClassLoader; // Transfer over InheritableThreadLocals. if (currentThread.inheritableValues != null) { inheritableValues = new ThreadLocal.Values(currentThread.inheritableValues); } // add ourselves to our ThreadGroup of choice this.group.addThread(this); }
public static Thread currentThread() { return VMThread.currentThread(); }
VmThread源码位于：
VMThread 的 currentThread 是一个 native 方法，其 JNI 实现为 android/dalvik/vm/native/java_lang_VMThread.cpp 中
static void Dalvik_java_lang_VMThread_currentThread(const u4* args, JValue* pResult) { UNUSED_PARAMETER(args); RETURN_PTR(dvmThreadSelf()->threadObj); }
这里有个dvmThreadSelf()方法：
Thread* dvmThreadSelf() { return (Thread*) pthread_getspecific(gDvm.pthreadKeySelf); }
可见这是一个存放在key为pthreadKeySelf的索引。
/* the java/lang/Thread that we are associated with */ Object* threadObj;
threadObj关联的就是android thread对象。
接着分析上面的代码，如果没有给新线程指定 group 那么就会指定 group 为当前线程所在的 group 中，然后给新线程设置 name，priority 等。最后通过调用 ThreadGroup 的 addThread 方法将新线程添加到 group 中：
/** * Called by the Thread constructor. */ final void addThread(Thread thread) throws IllegalThreadStateException { synchronized (threadRefs) { if (isDestroyed) { throw new IllegalThreadStateException(); } threadRefs.add(new WeakReference<Thread>(thread)); } }
threadRefs里面就是存放group对每一个thread的引用。
通过以上代码分析，thread构造方法仅仅只是设置了一些线程属性，并没有创建真正的线程。
Thread新创建的线程：
public synchronized void start() { checkNotStarted(); hasBeenStarted = true; VMThread.create(this, stackSize); }
Android Thread 的 start 方法很简单，仅仅是转调 VMThread 的 native 方法 create，其 JNI 实现为 android/dalvik/vm/native/java_lang_VMThread.cpp 中的 Dalvik_java_lang_VMThread_create 方法：
static void Dalvik_java_lang_VMThread_create(const u4* args, JValue* pResult) { Object* threadObj = (Object*) args[0]; s8 stackSize = GET_ARG_LONG(args, 1); /* copying collector will pin threadObj for us since it was an argument */ dvmCreateInterpThread(threadObj, (int) stackSize); RETURN_VOID(); }
dvmCreateInterpThread函数很长，但是它做了最重要的一件事：
bool dvmCreateInterpThread(Object* threadObj, int reqStackSize)
{
Thread* self = dvmThreadSelf();
Thread* newThread = allocThread(stackSize);
newThread->threadObj = threadObj;
Object* vmThreadObj = dvmAllocObject(gDvm.classJavaLangVMThread, ALLOC_DEFAULT);
dvmSetFieldInt(vmThreadObj, gDvm.offJavaLangVMThread_vmData, (u4)newThread);
dvmSetFieldObject(threadObj, gDvm.offJavaLangThread_vmThread, vmThreadObj);
pthread_t threadHandle;
int cc = pthread_create(&threadHandle, &threadAttr, interpThreadStart, newThread);
/*
* Tell the new thread to start.
*
* We must hold the thread list lock before messing with another thread.
* In the general case we would also need to verify that newThread was
* still in the thread list, but in our case the thread has not started
* executing user code and therefore has not had a chance to exit.
*
* We move it to VMWAIT, and it then shifts itself to RUNNING, which
* comes with a suspend-pending check.
*/
dvmLockThreadList(self);
assert(newThread->status == THREAD_STARTING);
newThread->status = THREAD_VMWAIT;
pthread_cond_broadcast(&gDvm.threadStartCond);
dvmUnlockThreadList();
}
/*
* Alloc and initialize a Thread struct.
*
* Does not create any objects, just stuff on the system (malloc) heap.
*/
static Thread* allocThread(int interpStackSize)
{
Thread* thread;
thread = (Thread*) calloc(1, sizeof(Thread));
thread->status = THREAD_INITIALIZING;
}
首先通过allocThread创建一个newThread的dalvik thread，并创建了一些属性。将设置其成员变量threadobj传入Android Thread threadobj.
创建vmThreadObj名字的Vmthread对象。
dvmSetFieldInt(vmThreadObj, gDvm.offJavaLangVMThread_vmData, (u4)newThread); dvmSetFieldObject(threadObj, gDvm.offJavaLangThread_vmThread, vmThreadObj); 把vmThreadObj的VM_data方法设置成newThread。
然后设置Android Thread vmThread变量为vmThreadObj。
这样通过vmThreadObj， Android Thread就和dalvik thread关联起来了。
然后就是
int cc = pthread_create(&threadHandle, &threadAttr, interpThreadStart, newThread); Yes，这个就是linux操作系统创建新线程的API接口！
接下来我们分析interpThreadStart,这是运行在新线程的入口。
/* * pthread entry function for threads started from interpreted code. */ static void* interpThreadStart(void* arg) { Thread* self = (Thread*) arg; std::string threadName(dvmGetThreadName(self)); setThreadName(threadName.c_str()); /* * Finish initializing the Thread struct. */ dvmLockThreadList(self); prepareThread(self); while (self->status != THREAD_VMWAIT) pthread_cond_wait(&gDvm.threadStartCond, &gDvm.threadListLock); dvmUnlockThreadList(); /* * Add a JNI context. */ self->jniEnv = dvmCreateJNIEnv(self); /* * Change our state so the GC will wait for us from now on. If a GC is * in progress this call will suspend us. */ dvmChangeStatus(self, THREAD_RUNNING); /* * Execute the "run" method. * * At this point our stack is empty, so somebody who comes looking for * stack traces right now won't have much to look at. This is normal. */ Method* run = self->threadObj->clazz->vtable[gDvm.voffJavaLangThread_run]; JValue unused; ALOGV("threadid=%d: calling run()", self->threadId); assert(strcmp(run->name, "run") == 0); dvmCallMethod(self, run, self->threadObj, &unused); ALOGV("threadid=%d: exiting", self->threadId); /* * Remove the thread from various lists, report its death, and free * its resources. */ dvmDetachCurrentThread(); return NULL; } /* * Finish initialization of a Thread struct. * * This must be called while executing in the new thread, but before the * thread is added to the thread list. * * NOTE: The threadListLock must be held by the caller (needed for * assignThreadId()). */ static bool prepareThread(Thread* thread) { assignThreadId(thread); thread->handle = pthread_self(); thread->systemTid = dvmGetSysThreadId(); setThreadSelf(thread); return true; } /* * Explore our sense of self. Stuffs the thread pointer into TLS. */ static void setThreadSelf(Thread* thread) { int cc; cc = pthread_setspecific(gDvm.pthreadKeySelf, thread); }
首先从Android Thread获得name，然后通过prepareThread设置线程的一些属性。并调用setThreadSelf方法，把dalvik thread放入TLS。
然后执行Android Thread的run方法。
public void run() { if (target != null) { target.run(); } }
至此，通过操作系统提供的接口，thread里面的run方法，在新线程中运行起来了！
本文参考：
1.《深入理解android内核设计思想》林学森
2.《Android内核剖析》
3.罗朝辉 http://www.cppblog.com/kesalin/archive/2014/07/11/android_thread_impl.html
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