jvm之方法区

方法区主要是存储类信息、常量的地方，也叫非堆。首先看一下官方文档对方法区的描述：

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.

简单来说，方法区是线程共享的，主要用来存储类的结构，比如：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。

虚拟机规范文档对方法区的要求挺宽泛的，实现或不实现GC、大小固定或者可以拓展、内存是否连续都可以。但jdk1.8中hotspot是实现了GC的，元空间直接分配在本地内存，自然也就是可拓展的了。

另外，jdk1.6->1.7->1.8这之间，hotspot方法区的变化还是很大的。在1.6中，方法区叫永久代；在1.7中把字符串常量池和静态变量移除到了堆中；1.8则直接改成了元空间并且直接分配在本地内存。