理解Android線程創建流程(轉)
/android/libcore/libart/src/main/java/java/lang/Thread.java
/art/runtime/native/java_lang_Thread.cc
/art/runtime/native/java_lang_Object.cc
/art/runtime/thread.cc
/system/core/libutils/Threads.cpp
/system/core/include/utils/AndroidThreads.h
/frameworks/base/core/jni/AndroidRuntime.cpp
一.概述
Android線程,一般地就是指Android虛擬機線程,而虛擬機線程由是通過系統調用而創建的Linux線程。純粹的Linux線程與虛擬機線程的區別在於虛擬機線程具有運行Java代碼的runtime. 除了虛擬機線程,還有Native線程,對於Native線程有分為是否具有訪問Java代碼的兩類線程。接下來,本文分析介紹這3類線程的創建過程。
二. Java線程
2.1 Thread.start
[-> Thread.java]
public synchronized void start() {
checkNotStarted(); //保證線程只有啟動一次
hasBeenStarted = true;
//[見流程2.2]
nativeCreate(this, stackSize, daemon);
}
nativeCreate
()這是一個native方法,那麽其所對應的JNI方法在哪呢?在java_lang_Thread.cc中通過gMethods是一個JNINativeMethod數組,其中一項為:
NATIVE_METHOD(Thread, nativeCreate, "(Ljava/lang/Thread;JZ)V"),
這裏的NATIVE_METHOD定義在java_lang_Object.cc文件,如下:
#define NATIVE_METHOD(className, functionName, signature) { #functionName, signature, reinterpret_cast<void*>(className ## _ ## functionName) }
將宏定義展開並代入,可得所對應的方法名為Thread_nativeCreate
2.2 Thread_nativeCreate
[-> java_lang_Thread.cc]
static void Thread_nativeCreate(JNIEnv* env, jclass, jobject java_thread,
jlong stack_size, jboolean daemon) {
//【見小節2.3】
Thread::CreateNativeThread(env, java_thread, stack_size, daemon == JNI_TRUE);
}
2.3 CreateNativeThread
[-> thread.cc]
void Thread::CreateNativeThread(JNIEnv* env, jobject java_peer, size_t stack_size, bool is_daemon) {
Thread* self = static_cast<JNIEnvExt*>(env)->self;
Runtime* runtime = Runtime::Current();
...
Thread* child_thread = new Thread(is_daemon);
child_thread->tlsPtr_.jpeer = env->NewGlobalRef(java_peer);
stack_size = FixStackSize(stack_size);
env->SetLongField(java_peer, WellKnownClasses::java_lang_Thread_nativePeer,
reinterpret_cast<jlong>(child_thread));
std::unique_ptr<JNIEnvExt> child_jni_env_ext(
JNIEnvExt::Create(child_thread, Runtime::Current()->GetJavaVM()));
int pthread_create_result = 0;
if (child_jni_env_ext.get() != nullptr) {
pthread_t new_pthread;
pthread_attr_t attr;
child_thread->tlsPtr_.tmp_jni_env = child_jni_env_ext.get();
//創建線程【見小節2.4】
pthread_create_result = pthread_create(&new_pthread,
&attr, Thread::CreateCallback, child_thread);
if (pthread_create_result == 0) {
child_jni_env_ext.release();
return;
}
}
...
}
2.4 pthread_create
pthread_create是pthread庫中的函數,通過syscall再調用到clone來請求內核創建線程。
- 原型:int pthread_create((pthread_t thread, pthread_attr_t *attr, void *(start_routine)(void *), void *arg)
- 頭文件:#include
- 輸入參數:thread:線程標識符; attr:線程屬性設置; start_routine:線程函數的起始地址; arg:傳遞給start_routine的參數;
- 返回值:成功則返回0;出錯則返回-1。
- 功能:創建線程,並調用線程起始地址所指向的函數start_routine。
關於pthread_create的分析,在後續Linux系列文章會再進一步深入分析。
三. Native線程(C/C++)
3.1 Thread.run
[-> Threads.cpp]
status_t Thread::run(const char* name, int32_t priority, size_t stack)
{
Mutex::Autolock _l(mLock);
//保證只會啟動一次
if (mRunning) {
return INVALID_OPERATION;
}
...
mRunning = true;
bool res;
if (mCanCallJava) {
//還能調用Java代碼的Native線程【見小節4.1】
res = createThreadEtc(_threadLoop,
this, name, priority, stack, &mThread);
} else {
//只能調用C/C++代碼的Native線程【見小節3.2】
res = androidCreateRawThreadEtc(_threadLoop,
this, name, priority, stack, &mThread);
}
if (res == false) {
...//清理
return UNKNOWN_ERROR;
}
return NO_ERROR;
}
mCanCallJava在Thread對象創建時,在構造函數中默認設置mCanCallJava=true.
- 當mCanCallJava=true,則代表創建的是不僅能調用C/C++代碼,還能能調用Java代碼的Native線程
- 當mCanCallJava=false,則代表創建的是只能調用C/C++代碼的Native線程。
3.2 androidCreateRawThreadEtc
[-> Threads.cpp]
int androidCreateRawThreadEtc(android_thread_func_t entryFunction,
void *userData,
const char* threadName __android_unused,
int32_t threadPriority,
size_t threadStackSize,
android_thread_id_t *threadId)
{
pthread_attr_t attr;
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
if (threadPriority != PRIORITY_DEFAULT || threadName != NULL) {
thread_data_t* t = new thread_data_t;
t->priority = threadPriority;
t->threadName = threadName ? strdup(threadName) : NULL;
t->entryFunction = entryFunction;
t->userData = userData;
entryFunction = (android_thread_func_t)&thread_data_t::trampoline;
userData = t;
}
if (threadStackSize) {
pthread_attr_setstacksize(&attr, threadStackSize);
}
errno = 0;
pthread_t thread;
//通過pthread_create創建線程
int result = pthread_create(&thread, &attr,
(android_pthread_entry)entryFunction, userData);
pthread_attr_destroy(&attr);
if (result != 0) {
... //創建失敗,則返回
return 0;
}
if (threadId != NULL) {
*threadId = (android_thread_id_t)thread;
}
return 1;
}
此處entryFunction所指向的是由[小節3.1]傳遞進來的,其值為_threadLoop。
3.3 _threadLoop
[-> Threads.cpp]
int Thread::_threadLoop(void* user)
{
//user是指Thread對象
Thread* const self = static_cast<Thread*>(user);
sp<Thread> strong(self->mHoldSelf);
wp<Thread> weak(strong);
self->mHoldSelf.clear();
//該參數對於gdb調試很有作用
self->mTid = gettid();
bool first = true;
do {
bool result;
if (first) {
first = false;
//首次運行時會調用readyToRun()做一些初始化準備工作
self->mStatus = self->readyToRun();
result = (self->mStatus == NO_ERROR);
if (result && !self->exitPending()) {
//
result = self->threadLoop();
}
} else {
result = self->threadLoop();
}
{
Mutex::Autolock _l(self->mLock);
if (result == false || self->mExitPending) {
self->mExitPending = true;
self->mRunning = false;
self->mThread = thread_id_t(-1);
self->mThreadExitedCondition.broadcast();
break;
}
}
strong.clear(); //釋放強引用
strong = weak.promote(); //重新請求強引用,用於下一次的循環
} while(strong != 0);
return 0;
}
不斷循環地調用成員方法threadLoop()。當滿足以下任一條件,則該線程將退出循環:
- 當前線程狀態存在錯誤,即mStatus != NO_ERROR;
- 當前線程即將退出, 即mExitPending = true; 調用Thread::requestExit()可觸發該過程。
- 當前線程的強引用釋放後,無法將弱引用提升成強引用的情況。
對於Native線程的實現方法,往往是通過繼承Thread對象,通過覆寫父類的readyToRun()和threadLoop()完成自定義線程的功能。
四. Native線程(Java)
4.1 createThreadEtc
[-> AndroidThreads.h]
inline bool createThreadEtc(thread_func_t entryFunction,
void *userData,
const char* threadName = "android:unnamed_thread",
int32_t threadPriority = PRIORITY_DEFAULT,
size_t threadStackSize = 0,
thread_id_t *threadId = 0)
{
//【見小節4.2】
return androidCreateThreadEtc(entryFunction, userData, threadName,
threadPriority, threadStackSize, threadId) ? true : false;
}
4.2 androidCreateThreadEtc
[-> Threads.cpp]
int androidCreateThreadEtc(android_thread_func_t entryFunction,
void *userData,
const char* threadName,
int32_t threadPriority,
size_t threadStackSize,
android_thread_id_t *threadId)
{
//【見小節4.3】
return gCreateThreadFn(entryFunction, userData, threadName,
threadPriority, threadStackSize, threadId);
}
此處gCreateThreadFn
默認指向androidCreateRawThreadEtc函數。 文章Android系統啟動-zygote篇的小節[3.3.1]已介紹 通過androidSetCreateThreadFunc()方法,gCreateThreadFn指向javaCreateThreadEtc函數。
4.3 javaCreateThreadEtc
[-> AndroidRuntime.cpp]
int AndroidRuntime::javaCreateThreadEtc(
android_thread_func_t entryFunction,
void* userData,
const char* threadName,
int32_t threadPriority,
size_t threadStackSize,
android_thread_id_t* threadId)
{
void** args = (void**) malloc(3 * sizeof(void*));
int result;
if (!threadName)
threadName = "unnamed thread";
args[0] = (void*) entryFunction;
args[1] = userData;
args[2] = (void*) strdup(threadName);
//【見小節4.4】
result = androidCreateRawThreadEtc(AndroidRuntime::javaThreadShell, args,
threadName, threadPriority, threadStackSize, threadId);
return result;
}
4.4 androidCreateRawThreadEtc
[-> Threads.cpp]
int androidCreateRawThreadEtc(android_thread_func_t entryFunction,
void *userData,
const char* threadName __android_unused,
int32_t threadPriority,
size_t threadStackSize,
android_thread_id_t *threadId)
{
...
if (threadStackSize) {
pthread_attr_setstacksize(&attr, threadStackSize);
}
...
//通過pthread_create創建線程
int result = pthread_create(&thread, &attr,
(android_pthread_entry)entryFunction, userData);
pthread_attr_destroy(&attr);
...
return 1;
}
此處entryFunction所指向的是由[小節4.3]傳遞進來的AndroidRuntime::javaThreadShell,接下來,進入該方法。
4.5 javaThreadShell
[-> AndroidRuntime.cpp]
int AndroidRuntime::javaThreadShell(void* args) {
void* start = ((void**)args)[0]; //指向_threadLoop
void* userData = ((void **)args)[1]; //線程對象
char* name = (char*) ((void **)args)[2]; //線程名
free(args);
JNIEnv* env;
int result;
//hook虛擬機【見小節4.5.1】
if (javaAttachThread(name, &env) != JNI_OK)
return -1;
// 調用_threadLoop()方法見小節4.5.2】
result = (*(android_thread_func_t)start)(userData);
//unhook虛擬機見小節4.5.3】
javaDetachThread();
free(name);
return result;
}
該方法主要功能:
- 調用javaAttachThread():將當前線程hook到當前進程所在的虛擬機,從而既能執行C/C++代碼,也能執行Java代碼。
- 調用_threadLoop():執行當前線程的核心邏輯代碼;
- 調用javaDetachThread():到此說明線程_threadLoop方法執行完成,則從當前進程的虛擬機中移除該線程。
4.5.1 javaAttachThread
[-> AndroidRuntime.cpp]
static int javaAttachThread(const char* threadName, JNIEnv** pEnv)
{
JavaVMAttachArgs args;
JavaVM* vm;
jint result;
vm = AndroidRuntime::getJavaVM();
args.version = JNI_VERSION_1_4;
args.name = (char*) threadName;
args.group = NULL;
// 將當前線程hook到當前進程所在的虛擬機
result = vm->AttachCurrentThread(pEnv, (void*) &args);
return result;
}
4.5.2 _threadLoop
[-> Threads.cpp]
int Thread::_threadLoop(void* user)
{
...
do {
if (first) {
...
self->mStatus = self->readyToRun();
result = (self->mStatus == NO_ERROR);
if (result && !self->exitPending()) {
result = self->threadLoop();
}
} else {
result = self->threadLoop();
}
Mutex::Autolock _l(self->mLock);
//當result=false則退出該線程
if (result == false || self->mExitPending) {
self->mExitPending = true;
self->mRunning = false;
self->mThread = thread_id_t(-1);
self->mThreadExitedCondition.broadcast();
break;
}
}
//釋放強引用,讓線程有機會退出
strong.clear();
//再次獲取強引用,用於下一輪循環
strong = weak.promote();
} while(strong != 0);
return 0;
}
該過程與【小節3.3】完全一致,見上文。
4.5.3 javaDetachThread
[-> AndroidRuntime.cpp]
static int javaDetachThread(void)
{
JavaVM* vm;
jint result;
vm = AndroidRuntime::getJavaVM();
//當前進程的虛擬機中移除該線程
result = vm->DetachCurrentThread();
return result;
}
在創建Native進程的整個過程,涉及到JavaVM的AttachCurrentThread和DetachCurrentThread方法,都已深入虛擬機內部原理,本文就先講到這裏,不再深入,後續有精力再深入研究虛擬機,準備寫一系列相關文章。
五. 總結
本文介紹了3類線程的創建過程,它們都有一個共同的特點,那就是真正的線程創建過程都是通過調用pthread_create
方法(見小節[2.3],[3.2],[4.4]),該方法經過層層調用,最終都會進入clone系統調用,這是linux創建線程或進程的通用接口。
Native線程中是否可以執行Java代碼的區別,在於通過javaThreadShell()方法從而實現在_threadLoop()執行前後增加分別將當前線程增加hook到虛擬機和從虛擬機移除的功能。調用過程:
- Native線程(Java版):該過程相對比較復雜,見如上流程圖:
- Native線程: 相對簡單,只有上圖中的紫色部分:thread.run -> androidCreateRawThreadEtc -> _threadLoop
- Java線程: Thread.start -> Thread_nativeCreate -> CreateNativeThread
轉自:http://gityuan.com/2016/09/24/android-thread/
理解Android線程創建流程(轉)