Android中art虚拟机启动流程

分类：Android, Java 阅读 (1,395)

7月 072020

转载请注明原文地址：http://bcoder.com/java/start-flow-of-arm-vm-in-android

本文基于Android8.1系统进行研究

一、启动zygote

在Linux内核启动完成后，首先启动系统的第一个进程init进程

init进程会读取init.rc中的配置文件

其中有Zygote的配置，init进程将启动zygote进程

zygote的入口在app_main.cpp中的main函数中

二、解析传入参数，调用AndroidRuntime的start方法

在app_main.cpp中的main函数中，首先解析传入的相关参数，并通过如下代码进入ZygoteInit的main函数中

    if (zygote) {
        runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
    }

if (zygote) {

runtime.start("com.android.internal.os.ZygoteInit", args, zygote);

}

runtime是AppRuntime的一个实例，AppRuntim继承自AndroidRuntime

start函数定义在AndroidRuntime中，下面是start函数的注释

/*
 * Start the Android runtime.  This involves starting the virtual machine
 * and calling the "static void main(String[] args)" method in the class
 * named by "className".
 *
 * Passes the main function two arguments, the class name and the specified
 * options string.
*
* 翻译：
* 启动Android运行时，包括启动虚拟机，调用className参数对应的main函数
 */

* Start the Android runtime. This involves starting the virtual machine

* and calling the "static void main(String[] args)" method in the class

* named by "className".

* Passes the main function two arguments, the class name and the specified

* options string.

* 翻译：

* 启动Android运行时，包括启动虚拟机，调用className参数对应的main函数

start函数中的这段代码切入到了startVm函数中

    /* start the virtual machine */
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    JNIEnv* env;
    if (startVm(&mJavaVM, &env, zygote) != 0) {
        return;
    }
    onVmCreated(env);

/* start the virtual machine */

JniInvocation jni_invocation;

jni_invocation.Init(NULL);

JNIEnv* env;

if (startVm(&mJavaVM, &env, zygote) != 0) {

return;

}

onVmCreated(env);

三、startVm函数

这个函数前边也是一堆处理Vm启动参数的逻辑

在函数的最好调用JNI_CreateJavaVM函数通过c++层的库创建虚拟机

四、JNI_CreateJavaVM函数

JNI_CreateJavaVM函数是在art/runtime/java_vm_ext.cc中实现的，代码如下：

// JNI Invocation interface.

extern "C" jint JNI_CreateJavaVM(JavaVM** p_vm, JNIEnv** p_env, void* vm_args) {
  ScopedTrace trace(__FUNCTION__);
  const JavaVMInitArgs* args = static_cast<JavaVMInitArgs*>(vm_args);
  if (JavaVMExt::IsBadJniVersion(args->version)) {
    LOG(ERROR) << "Bad JNI version passed to CreateJavaVM: " << args->version;
    return JNI_EVERSION;
  }
  RuntimeOptions options;
  for (int i = 0; i < args->nOptions; ++i) {
    JavaVMOption* option = &args->options[i];
    options.push_back(std::make_pair(std::string(option->optionString), option->extraInfo));
  }
  bool ignore_unrecognized = args->ignoreUnrecognized;
  if (!Runtime::Create(options, ignore_unrecognized)) {
    return JNI_ERR;
  }

  // Initialize native loader. This step makes sure we have
  // everything set up before we start using JNI.
  android::InitializeNativeLoader();

  Runtime* runtime = Runtime::Current();
  bool started = runtime->Start();
  if (!started) {
    delete Thread::Current()->GetJniEnv();
    delete runtime->GetJavaVM();
    LOG(WARNING) << "CreateJavaVM failed";
    return JNI_ERR;
  }

  *p_env = Thread::Current()->GetJniEnv();
  *p_vm = runtime->GetJavaVM();
  return JNI_OK;
}

// JNI Invocation interface.

extern "C" jint JNI_CreateJavaVM(JavaVM** p_vm, JNIEnv** p_env, void* vm_args) {

ScopedTrace trace(__FUNCTION__);

const JavaVMInitArgs* args = static_cast<JavaVMInitArgs*>(vm_args);

if (JavaVMExt::IsBadJniVersion(args->version)) {

LOG(ERROR) << "Bad JNI version passed to CreateJavaVM: " << args->version;

return JNI_EVERSION;

}

RuntimeOptions options;

for (int i = 0; i < args->nOptions; ++i) {

JavaVMOption* option = &args->options[i];

options.push_back(std::make_pair(std::string(option->optionString), option->extraInfo));

}

bool ignore_unrecognized = args->ignoreUnrecognized;

if (!Runtime::Create(options, ignore_unrecognized)) {

return JNI_ERR;

}

// Initialize native loader. This step makes sure we have

// everything set up before we start using JNI.

android::InitializeNativeLoader();

Runtime* runtime = Runtime::Current();

bool started = runtime->Start();

if (!started) {

delete Thread::Current()->GetJniEnv();

delete runtime->GetJavaVM();

LOG(WARNING) << "CreateJavaVM failed";

return JNI_ERR;

}

*p_env = Thread::Current()->GetJniEnv();

*p_vm = runtime->GetJavaVM();

return JNI_OK;

}

Runtime是在art/runtime/runtime.h中定义的类

Runtime::Current()返回一个静态的Runtime实例，代码如下：

static Runtime* instance_;

1	static Runtime* instance_;

runtime-Start()启动虚拟机

五、其他参考文章

Android ART运行时无缝替换Dalvik虚拟机的过程分析

https://blog.csdn.net/zhu929033262/article/details/77053640

Android虚拟机art流程：从zygote开始梳理art的启动（1）

https://www.shennongblog.com/art-zygote/

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