Java源代码是如何编译，加载到内存中的？

2023-08-09 16:01 由 darcy_yuan 发表于 #后端开发

1.前言

相信许多开发同学看过《深入理解java虚拟机》，也阅读过java虚拟机规范，书籍和文档给人的感觉不够直观，本文从一个简单的例子来看看jvm是如何工作的吧。

本文所有操作均在mac上进行。

2.示例代码

示例代码采用最常见的双重检索单例模式：

package interview.desginpattern.singletion.doublecheck;

import java.io.Serializable;

public class Singleton implements Serializable {
    private static volatile Singleton instance = null;

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }

        return instance;
    }
}

3.指令

经过编译后，我们得到class文件，然后用javap命令查看相关指令

javap -c Singleton：

public class interview.desginpattern.singletion.doublecheck.Singleton implements java.io.Serializable {
  public static interview.desginpattern.singletion.doublecheck.Singleton getInstance();
    Code:
       0: getstatic     #2                  // get static instance
       3: ifnonnull     37                  // instance is null，jump 37
       6: ldc           #3                  // push class Singleton to operand stack
       8: dup                               // Duplicate the top (class Singleton) operand stack value
       9: astore_0                          // Store reference(class Singleton) into local variable
      10: monitorenter                      // synchronized start
      11: getstatic     #2                  
      14: ifnonnull     27                  // instance is null，jump 27
      17: new           #3                  // new class interview/desginpattern/singletion/doublecheck/Singleton
      20: dup                               
      21: invokespecial #4                  // Method "<init>":()V
      24: putstatic     #2                  
      27: aload_0                           // Load reference (class Singleton) from local variable
      28: monitorexit                       // synchronized end
      29: goto          37
      32: astore_1
      33: aload_0
      34: monitorexit                       // synchronized end (exception)
      35: aload_1
      36: athrow                            // throw exception
      37: getstatic     #2                  
      40: areturn
    Exception table:
       from    to  target type
          11    29    32   any
          32    35    32   any

  static {};                                // static code
    Code:
       0: aconst_null                       // Push the null object reference onto the operand stack
       1: putstatic     #2                  // set static instance
       4: return
}

有以下几点需要说明：