标签: java

1. 架构

JVM 的软件架构如下：

JAVA 内存布局如下图所示：

通过对典型应用的 JVM 内存进行拆解，如下：

Total: reserved=15538605KB, committed=14221757KB

Java Heap (reserved=9601024KB, committed=9601024KB)
Class (reserved=2095827KB, committed=1215827KB)    
    Metadata (reserved=995328KB, committed=993536KB) 
    ## 每个类的元数据：类名、方法、字段描述    

    Class space (reserved=1048576KB, committed=170368KB) 
    ## 为类加载器存储的数据预留的空间

Thread (reserved=43505KB, committed=10101KB) 
## 包括线程栈和线程本地存储    
    stack (reserved=43505KB, committed=10101KB) 
    ## 对每个线程分配的栈空间，大小可通过 -Xss 配置

Code (reserved=977086KB, committed=573642KB) # 为 JIT 编译器预留的内存
GC (reserved=495136KB, committed=495136KB) # 为 GC 预留的内存
Compiler (reserved=734KB, committed=734KB)
Internal (reserved=20654KB, committed=20654KB) #JVM 内部使用的内存，依赖于 JVM 的实现
Other (reserved=733088KB, committed=733088KB) 
## Native Memory Tracking, Arena Chunk, Logging, 
## Arguments, Module,Synchronizer, Safepoint, Wisp, null

JVM Inst

Abstract

JAVA 字符串操作的优化。

Java 对象布局

Java Array 对象布局

Java Char Array
Head	Mark Word	4 Bytes
	Class Pointer	4 Bytes
	Length of Array	Offset = 8
Instance Data	Data of Array
Padding

ART Create

Abstract

Art 的创建过程是一个很复杂的命题，所以我们单独开设一章来对这个过程进行学习。

@todo 增加全局的流程图。

Art Create

JNI_CreateJavaVM

当我们选择了 ART 运行时, Zygote 进程在启动过程中，会调用 libart.so 里面的函数 JNI_CreateVM 来创建一个 art 虚拟机，这个函数的实现如下：

// art/runtime/jni/java_vm_ext.cc
// 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;
}