JVM之内存模型

• 内存模型-私有部分
  
  
  
  
  代码演示：

  package com.imooc.java.reflect;
  
  public class ByteCodeSample {
     public static int add(int a,int b){
         int c = 0;
         c = a+b;
         return c;
     }
  }
  
  

  对class文件进行反编译

  appledeMBP:com apple$ javac imooc/java/reflect/ByteCodeSample.java 
  appledeMBP:com apple$ javap -verbose imooc/java/reflect/ByteCodeSample.class 
  Classfile /Users/apple/Desktop/demo/SparkTest/src/main/java/com/imooc/java/reflect/ByteCodeSample.class
    Last modified 2019-4-9; size 293 bytes
    MD5 checksum e529fd08d9eec77927dd36bdf7040d12
    Compiled from "ByteCodeSample.java"
  public class com.imooc.java.reflect.ByteCodeSample
    minor version: 0
    major version: 52
    flags: ACC_PUBLIC, ACC_SUPER
  Constant pool:
     #1 = Methodref          #3.#12         // java/lang/Object."<init>":()V
     #2 = Class              #13            // com/imooc/java/reflect/ByteCodeSample
     #3 = Class              #14            // java/lang/Object
     #4 = Utf8               <init>
     #5 = Utf8               ()V
     #6 = Utf8               Code
     #7 = Utf8               LineNumberTable
     #8 = Utf8               add
     #9 = Utf8               (II)I
    #10 = Utf8               SourceFile
    #11 = Utf8               ByteCodeSample.java
    #12 = NameAndType        #4:#5          // "<init>":()V
    #13 = Utf8               com/imooc/java/reflect/ByteCodeSample
    #14 = Utf8               java/lang/Object
  {
    public com.imooc.java.reflect.ByteCodeSample();
      descriptor: ()V
      flags: ACC_PUBLIC
      Code:
        stack=1, locals=1, args_size=1
           0: aload_0
           1: invokespecial #1                  // Method java/lang/Object."<init>":()V
           4: return
        LineNumberTable:
          line 3: 0
  
    public static int add(int, int);
      descriptor: (II)I
      flags: ACC_PUBLIC, ACC_STATIC
      Code:
        stack=2, locals=3, args_size=2
           0: iconst_0
           1: istore_2
           2: iload_0
           3: iload_1
           4: iadd
           5: istore_2
           6: iload_2
           7: ireturn
        LineNumberTable:
          line 5: 0
          line 6: 2
          line 7: 6
  }
  SourceFile: "ByteCodeSample.java"
  
  

• 内存模型-公有部分
  jdk1.8将永久代(PermGen)也就是方法区换成了元空间(MetaSpace)
  区别是：元空间使用本地内存，而永久代使用的是jvm的内存
  优势：1、字符串常量池存在永久代中，容易出现性能问题和内存溢出
  2、类和方法的信息大小难确定，给永久代的大小指定带来困难
  3、永久代会为GC带来不必要的复杂性

• 常见面试题
  1、JVM三大性能调优参数 -Xms -Xmx -Xss的含义