java虚拟机主要由三块组成：
- 类装载子系统运行时数据区（内存模型）字节码执行引擎
java指令运行编译后的字节码文件，首先通过类装载子系统（由C++实现）把字节码文件放入运行时数据区（内存模型）里面，然后再通过字节码执行引擎（由C++实现）来执行代码，其中最核心的是运行时数据区，jvm调优只要优化运行时数据区。
Oracle官方解释
运行时数据区：The Java Virtual Machine defines various run-time data areas that are used during execution of a program. Some of these data areas are created on Java Virtual Machine start-up and are destroyed only when the Java Virtual Machine exits. Other data areas are per thread. Per-thread data areas are created when a thread is created and destroyed when the thread exits.java（虚拟机定义了在程序执行期间使用的各种运行时数据区域。其中一些数据区是在Java虚拟机启动时创建的，只有java虚拟机退出时才会别销毁。其他数据区域是每个线程。每个线程的数据区域在创建线程时创建，并在线程退出时销毁）。
程序计数器：The Java Virtual Machine can support many threads of execution at once (JLS §17). Each Java Virtual Machine thread has its own pc (program counter) register. At any point, each Java Virtual Machine thread is executing the code of a single method, namely the current method (§2.6) for that thread. If that method is not native, the pc register contains the address of the Java Virtual Machine instruction currently being executed. If the method currently being executed by the thread is native, the value of the Java Virtual Machine’s pc register is undefined. The Java Virtual Machine’s pc register is wide enough to hold a returnAddress or a native pointer on the specific platform.（Java虚拟机可以同时支持多个执行线程。每个java虚拟机线程都有自己的pc（程序寄存器）寄存器。在任何时候，每个java虚拟机线程都在执行单个方法的代码，即该线程的当前方法。如果该方法不是native，则pc寄存器包含当前正在执行的java虚拟机指令的地址。如果线程当前正在执行的方法是native，则java虚拟机的pc寄存器的值是未定义的。java虚拟机的pc寄存器的宽度足以容纳特定平台上的返回地址或本机指针）
栈（线程、虚拟机栈）：Each Java Virtual Machine thread has a private Java Virtual Machine stack, created at the same time as the thread. A Java Virtual Machine stack stores frames (§2.6). A Java Virtual Machine stack is analogous to the stack of a conventional language such as C: it holds local variables and partial results, and plays a part in method invocation and return. Because the Java Virtual Machine stack is never manipulated directly except to push and pop frames, frames may be heap allocated. The memory for a Java Virtual Machine stack does not need to be contiguous.
In the First Edition of The Java® Virtual Machine Specification, the Java Virtual Machine stack was known as the Java stack.
This specification permits Java Virtual Machine stacks either to be of a fixed size or to dynamically expand and contract as required by the computation. If the Java Virtual Machine stacks are of a fixed size, the size of each Java Virtual Machine stack may be chosen independently when that stack is created.
A Java Virtual Machine implementation may provide the programmer or the user control over the initial size of Java Virtual Machine stacks, as well as, in the case of dynamically expanding or contracting Java Virtual Machine stacks, control over the maximum and minimum sizes.
The following exceptional conditions are associated with Java Virtual Machine stacks:
If the computation in a thread requires a larger Java Virtual Machine stack than is permitted, the Java Virtual Machine throws a StackOverflowError.
If Java Virtual Machine stacks can be dynamically expanded, and expansion is attempted but insufficient memory can be made available to effect the expansion, or if insufficient memory can be made available to create the initial Java Virtual Machine stack for a new thread, the Java Virtual Machine throws an OutOfMemoryError.（
Each Java Virtual Machine thread has a private Java Virtual Machine stack, created at the same time as the thread. A Java Virtual Machine stack stores frames (§2.6). A Java Virtual Machine stack is analogous to the stack of a conventional language such as C: it holds local variables and partial results, and plays a part in method invocation and return. Because the Java Virtual Machine stack is never manipulated directly except to push and pop frames, frames may be heap allocated. The memory for a Java Virtual Machine stack does not need to be contiguous.
In the First Edition of The Java® Virtual Machine Specification, the Java Virtual Machine stack was known as the Java stack.
This specification permits Java Virtual Machine stacks either to be of a fixed size or to dynamically expand and contract as required by the computation. If the Java Virtual Machine stacks are of a fixed size, the size of each Java Virtual Machine stack may be chosen independently when that stack is created.
A Java Virtual Machine implementation may provide the programmer or the user control over the initial size of Java Virtual Machine stacks, as well as, in the case of dynamically expanding or contracting Java Virtual Machine stacks, control over the maximum and minimum sizes.
The following exceptional conditions are associated with Java Virtual Machine stacks:
If the computation in a thread requires a larger Java Virtual Machine stack than is permitted, the Java Virtual Machine throws a StackOverflowError.
If Java Virtual Machine stacks can be dynamically expanded, and expansion is attempted but insufficient memory can be made available to effect the expansion, or if insufficient memory can be made available to create the initial Java Virtual Machine stack for a new thread, the Java Virtual Machine throws an OutOfMemoryError.（
每个Java虚拟机线程都有一个私有Java虚拟机堆栈，与线程同时创建。Java虚拟机堆栈存储帧。Java虚拟机堆栈类似于C等传统语言的堆栈：它保存局部变量和部分结果，并在方法调用和返回中发挥作用。由于Java虚拟机堆栈除了用于推送和弹出帧之外，从未被直接操作，因此帧可能是堆分配的。Java虚拟机堆栈的内存不需要是连续的。
在Java®虚拟机规范的第一版中，Java虚拟机堆栈被称为Java堆栈。
该规范允许Java虚拟机堆栈具有固定大小，或者根据计算需要动态扩展和收缩。如果Java虚拟机堆栈的大小是固定的，那么在创建该堆栈时，可以独立选择每个Java虚拟机堆栈的大小。
Java虚拟机实现可以为程序员或用户提供对Java虚拟机堆栈初始大小的控制，以及在动态扩展或收缩Java虚拟机堆栈的情况下，对最大和最小大小的控制。
以下异常情况与Java虚拟机堆栈相关：
如果线程中的计算需要比允许的更大的Java虚拟机堆栈，Java虚拟机将抛出StackOverflower错误。
如果Java虚拟机堆栈可以动态扩展，并且尝试了扩展，但是没有足够的内存来实现扩展，或者如果没有足够的内存来创建新线程的初始Java虚拟机堆栈，Java虚拟机将抛出OutOfMemory错误。）
堆：The Java Virtual Machine has a heap that is shared among all Java Virtual Machine threads. The heap is the run-time data area from which memory for all class instances and arrays is allocated.
The heap is created on virtual machine start-up. Heap storage for objects is reclaimed by an automatic storage management system (known as a garbage collector); objects are never explicitly deallocated. The Java Virtual Machine assumes no particular type of automatic storage management system, and the storage management technique may be chosen according to the implementor’s system requirements. The heap may be of a fixed size or may be expanded as required by the computation and may be contracted if a larger heap becomes unnecessary. The memory for the heap does not need to be contiguous.
A Java Virtual Machine implementation may provide the programmer or the user control over the initial size of the heap, as well as, if the heap can be dynamically expanded or contracted, control over the maximum and minimum heap size.
The following exceptional condition is associated with the heap:
If a computation requires more heap than can be made available by the automatic storage management system, the Java Virtual Machine throws an OutOfMemoryError.（每个Java虚拟机线程都有一个私有Java虚拟机堆栈，与线程同时创建。Java虚拟机堆栈存储帧（§2.6）。Java虚拟机堆栈类似于C等传统语言的堆栈：它保存局部变量和部分结果，并在方法调用和返回中发挥作用。由于Java虚拟机堆栈除了用于推送和弹出帧之外，从未被直接操作，因此帧可能是堆分配的。Java虚拟机堆栈的内存不需要是连续的。
在Java®虚拟机规范的第一版中，Java虚拟机堆栈被称为Java堆栈。
该规范允许Java虚拟机堆栈具有固定大小，或者根据计算需要动态扩展和收缩。如果Java虚拟机堆栈的大小是固定的，那么在创建该堆栈时，可以独立选择每个Java虚拟机堆栈的大小。
Java虚拟机实现可以为程序员或用户提供对Java虚拟机堆栈初始大小的控制，以及在动态扩展或收缩Java虚拟机堆栈的情况下，对最大和最小大小的控制。
以下异常情况与Java虚拟机堆栈相关：
如果线程中的计算需要比允许的更大的Java虚拟机堆栈，Java虚拟机将抛出StackOverflower错误。
如果Java虚拟机堆栈可以动态扩展，并且尝试了扩展，但是没有足够的内存来实现扩展，或者如果没有足够的内存来创建新线程的初始Java虚拟机堆栈，Java虚拟机将抛出OutOfMemory错误。）
方法区（元空间）：The Java Virtual Machine has a method area that is shared among all Java Virtual Machine threads. The method area is analogous to the storage area for compiled code of a conventional language or analogous to the “text” segment in an operating system process. It stores per-class structures such as the run-time constant pool, field and method data, and the code for methods and constructors, including the special methods (§2.9) used in class and instance initialization and interface initialization.
The method area is created on virtual machine start-up. Although the method area is logically part of the heap, simple implementations may choose not to either garbage collect or compact it. This specification does not mandate the location of the method area or the policies used to manage compiled code. The method area may be of a fixed size or may be expanded as required by the computation and may be contracted if a larger method area becomes unnecessary. The memory for the method area does not need to be contiguous.
A Java Virtual Machine implementation may provide the programmer or the user control over the initial size of the method area, as well as, in the case of a varying-size method area, control over the maximum and minimum method area size.
The following exceptional condition is associated with the method area:
If memory in the method area cannot be made available to satisfy an allocation request, the Java Virtual Machine throws an OutOfMemoryError.（Java虚拟机有一个在所有Java虚拟机线程之间共享的方法区域。方法区类似于常规语言编译代码的存储区，或类似于操作系统进程中的“文本”段。它存储每类结构，例如运行时常量池、字段和方法数据，以及方法和构造函数的代码，包括类和实例初始化以及接口初始化中使用的特殊方法（§2.9）。
方法区域是在虚拟机启动时创建的。尽管方法区域在逻辑上是堆的一部分，但简单的实现可能会选择不进行垃圾收集或压缩。本规范不要求指定方法区域的位置或用于管理已编译代码的策略。方法面积可以是固定大小，也可以根据计算要求进行扩展，如果不需要更大的方法面积，可以缩小。方法区域的内存不需要是连续的。
Java虚拟机实现可以为程序员或用户提供对方法区域初始大小的控制，以及在方法区域大小变化的情况下，对最大和最小方法区域大小的控制。
以下异常情况与方法区域有关：
如果方法区域中的内存无法满足分配请求，Java虚拟机将抛出OutOfMemoryError。）
运行时常量池：A run-time constant pool is a per-class or per-interface run-time representation of the constant_pool table in a class file (§4.4). It contains several kinds of constants, ranging from numeric literals known at compile-time to method and field references that must be resolved at run-time. The run-time constant pool serves a function similar to that of a symbol table for a conventional programming language, although it contains a wider range of data than a typical symbol table.
Each run-time constant pool is allocated from the Java Virtual Machine’s method area (§2.5.4). The run-time constant pool for a class or interface is constructed when the class or interface is created (§5.3) by the Java Virtual Machine.
The following exceptional condition is associated with the construction of the run-time constant pool for a class or interface:
When creating a class or interface, if the construction of the run-time constant pool requires more memory than can be made available in the method area of the Java Virtual Machine, the Java Virtual Machine throws an OutOfMemoryError.
See §5 (Loading, linking, and Initializing) for information about the construction of the run-time constant pool.（运行时常量池是类文件中常量池表的每类或每接口运行时表示形式（§4.4）。它包含几种常量，从编译时已知的数字文本到必须在运行时解析的方法和字段引用。运行时常量池的功能类似于传统编程语言的符号表，尽管它包含比典型符号表更广泛的数据范围。
每个运行时常量池都是从Java虚拟机的方法区域分配的（§2.5.4）。类或接口的运行时常量池是在Java虚拟机创建类或接口时构造的（§5.3）。
以下异常情况与类或接口的运行时常量池的构造有关：
创建类或接口时，如果运行时常量池的构造需要的内存超过Java虚拟机的方法区域中可用的内存，Java虚拟机会抛出OutOfMemoryError。
有关运行时常量池构造的信息，请参见§5（加载、链接和初始化）。）
本机方法堆栈：An implementation of the Java Virtual Machine may use conventional stacks, colloquially called “C stacks,” to support native methods (methods written in a language other than the Java programming language). Native method stacks may also be used by the implementation of an interpreter for the Java Virtual Machine’s instruction set in a language such as C. Java Virtual Machine implementations that cannot load native methods and that do not themselves rely on conventional stacks need not supply native method stacks. If supplied, native method stacks are typically allocated per thread when each thread is created.
This specification permits native method stacks either to be of a fixed size or to dynamically expand and contract as required by the computation. If the native method stacks are of a fixed size, the size of each native method stack may be chosen independently when that stack is created.
A Java Virtual Machine implementation may provide the programmer or the user control over the initial size of the native method stacks, as well as, in the case of varying-size native method stacks, control over the maximum and minimum method stack sizes.
The following exceptional conditions are associated with native method stacks:
If the computation in a thread requires a larger native method stack than is permitted, the Java Virtual Machine throws a StackOverflowError.
If native method stacks can be dynamically expanded and native method stack expansion is attempted but insufficient memory can be made available, or if insufficient memory can be made available to create the initial native method stack for a new thread, the Java Virtual Machine throws an OutOfMemoryError.（Java虚拟机的实现可以使用传统堆栈（俗称“C堆栈”）来支持本机方法（用Java编程语言以外的语言编写的方法）。本机方法栈也可以由解释器的实现来使用，解释器用于Java虚拟机的指令集，其语言为C。无法加载本机方法且自身不依赖于传统栈的Java虚拟机实现不需要提供本机方法栈。如果提供，则在创建每个线程时，通常会为每个线程分配本机方法堆栈。
本规范允许本机方法堆栈具有固定大小，或者根据计算需要动态扩展和收缩。如果本机方法堆栈的大小固定，则在创建该堆栈时，可以独立选择每个本机方法堆栈的大小。
Java虚拟机实现可以为程序员或用户提供对本机方法堆栈初始大小的控制，以及在本机方法堆栈大小不同的情况下，对最大和最小方法堆栈大小的控制。
以下异常情况与本机方法堆栈相关：
如果线程中的计算需要比允许的更大的本机方法堆栈，Java虚拟机将抛出StackOverflower错误。
如果可以动态扩展本机方法堆栈，并尝试扩展本机方法堆栈，但内存不足，或者如果内存不足，无法为新线程创建初始本机方法堆栈，Java虚拟机将抛出OutOfMemory错误。）
栈（线程栈）：
栈帧
