MartinDelophy

浅谈 node-jvm(2)

2018-5-15 by MartinDelophy

上回，我们很简单的对node-jvm进行了一定的分析，但问题还是停留在表面，仅仅只是介绍了node 里面的一些特点，和java并无很多的关系，今天让我们接着来讲后续的内容。 要想很清晰的知道后续的内容，我们首先要对java class类文件做一定的了解。 我想在网上这样的文章算是比较多了，我推荐几个链接可以去看看 1.https://blog.csdn.net/u010425776/article/details/51245055 2.https://www.cnblogs.com/Qian123/p/5707562.html 3.https://blog.csdn.net/u013309870/article/details/72975536 4.http://www.blogjava.net/DLevin/archive/2011/09/05/358033.html 5.http://www.blogjava.net/DLevin/archive/2011/09/05/358034.html

看了上面几个文章之后，我们开始实战来分析一下下面的代码 首先看到的是用来针对fib 进行计算的java文件

Main.java

public class Main {

    public static long fib(int n) {
        if (n <= 1)
            return n;
        return fib(n - 1) + fib(n - 2);
    }

    public static void main(String[] args) {
        if (args.length == 0) {
            System.out.print("help: java Fibonacci {Number}");
            return;
        }

        int N = Integer.parseInt(args[0]);

        long start = System.currentTimeMillis();
        System.out.format("Fibonacci from 1 to %s:\n", N);
        for (int i = 1; i <= N; i++) {
            System.out.println(i + ": " + fib(i));
        }
        long stop = System.currentTimeMillis();
        System.out.println("time: " + (stop - start) + "ms");

        System.out.println("done.");
    }
}

该部分代码，主要是实现了最基本的fib 功能，并且调用。

那么接下来我用make 生成Main.class文件,然后通过 javap -v -verbose Main 分析一下改该类文件中的内容，内容如下：

Last modified May 7, 2018; size 1286 bytes
  MD5 checksum 115352bb394b31575a6288f7bdf82f35
  Compiled from "Main.java"
public class Main
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #9.#35         // java/lang/Object."<init>":()V
   #2 = Methodref          #23.#36        // Main.fib:(I)J
   #3 = Fieldref           #37.#38        // java/lang/System.out:Ljava/io/PrintStream;
   #4 = String             #39            // help: java Fibonacci {Number}
   #5 = Methodref          #40.#41        // java/io/PrintStream.print:(Ljava/lang/String;)V
   #6 = Methodref          #42.#43        // java/lang/Integer.parseInt:(Ljava/lang/String;)I
   #7 = Methodref          #37.#44        // java/lang/System.currentTimeMillis:()J
   #8 = String             #45            // Fibonacci from 1 to %s:\n
   #9 = Class              #46            // java/lang/Object
  #10 = Methodref          #42.#47        // java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
  #11 = Methodref          #40.#48        // java/io/PrintStream.format:(Ljava/lang/String;[Ljava/lang/Object;)Ljava/io/PrintStream;
  #12 = Class              #49            // java/lang/StringBuilder
  #13 = Methodref          #12.#35        // java/lang/StringBuilder."<init>":()V
  #14 = Methodref          #12.#50        // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
  #15 = String             #51            // :
  #16 = Methodref          #12.#52        // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #17 = Methodref          #12.#53        // java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder;
  #18 = Methodref          #12.#54        // java/lang/StringBuilder.toString:()Ljava/lang/String;
  #19 = Methodref          #40.#55        // java/io/PrintStream.println:(Ljava/lang/String;)V
  #20 = String             #56            // time:
  #21 = String             #57            // ms
  #22 = String             #58            // done.
  #23 = Class              #59            // Main
  #24 = Utf8               <init>
  #25 = Utf8               ()V
  #26 = Utf8               Code
  #27 = Utf8               LineNumberTable
  #28 = Utf8               fib
  #29 = Utf8               (I)J
  #30 = Utf8               StackMapTable
  #31 = Utf8               main
  #32 = Utf8               ([Ljava/lang/String;)V
  #33 = Utf8               SourceFile
  #34 = Utf8               Main.java
  #35 = NameAndType        #24:#25        // "<init>":()V
  #36 = NameAndType        #28:#29        // fib:(I)J
  #37 = Class              #60            // java/lang/System
  #38 = NameAndType        #61:#62        // out:Ljava/io/PrintStream;
  #39 = Utf8               help: java Fibonacci {Number}
  #40 = Class              #63            // java/io/PrintStream
  #41 = NameAndType        #64:#65        // print:(Ljava/lang/String;)V
  #42 = Class              #66            // java/lang/Integer
  #43 = NameAndType        #67:#68        // parseInt:(Ljava/lang/String;)I
  #44 = NameAndType        #69:#70        // currentTimeMillis:()J
  #45 = Utf8               Fibonacci from 1 to %s:\n
  #46 = Utf8               java/lang/Object
  #47 = NameAndType        #71:#72        // valueOf:(I)Ljava/lang/Integer;
  #48 = NameAndType        #73:#74        // format:(Ljava/lang/String;[Ljava/lang/Object;)Ljava/io/PrintStream;
  #49 = Utf8               java/lang/StringBuilder
  #50 = NameAndType        #75:#76        // append:(I)Ljava/lang/StringBuilder;
  #51 = Utf8               :
  #52 = NameAndType        #75:#77        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #53 = NameAndType        #75:#78        // append:(J)Ljava/lang/StringBuilder;
  #54 = NameAndType        #79:#80        // toString:()Ljava/lang/String;
  #55 = NameAndType        #81:#65        // println:(Ljava/lang/String;)V
  #56 = Utf8               time:
  #57 = Utf8               ms
  #58 = Utf8               done.
  #59 = Utf8               Main
  #60 = Utf8               java/lang/System
  #61 = Utf8               out
  #62 = Utf8               Ljava/io/PrintStream;
  #63 = Utf8               java/io/PrintStream
  #64 = Utf8               print
  #65 = Utf8               (Ljava/lang/String;)V
  #66 = Utf8               java/lang/Integer
  #67 = Utf8               parseInt
  #68 = Utf8               (Ljava/lang/String;)I
  #69 = Utf8               currentTimeMillis
  #70 = Utf8               ()J
  #71 = Utf8               valueOf
  #72 = Utf8               (I)Ljava/lang/Integer;
  #73 = Utf8               format
  #74 = Utf8               (Ljava/lang/String;[Ljava/lang/Object;)Ljava/io/PrintStream;
  #75 = Utf8               append
  #76 = Utf8               (I)Ljava/lang/StringBuilder;
  #77 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  #78 = Utf8               (J)Ljava/lang/StringBuilder;
  #79 = Utf8               toString
  #80 = Utf8               ()Ljava/lang/String;
  #81 = Utf8               println
{
  public Main();
    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 1: 0

  public static long fib(int);
    descriptor: (I)J
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=4, locals=1, args_size=1
         0: iload_0
         1: iconst_1
         2: if_icmpgt     8
         5: iload_0
         6: i2l
         7: lreturn
         8: iload_0
         9: iconst_1
        10: isub
        11: invokestatic  #2                  // Method fib:(I)J
        14: iload_0
        15: iconst_2
        16: isub
        17: invokestatic  #2                  // Method fib:(I)J
        20: ladd
        21: lreturn
      LineNumberTable:
        line 4: 0
        line 5: 5
        line 6: 8
      StackMapTable: number_of_entries = 1
        frame_type = 8 /* same */

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=6, locals=6, args_size=1
         0: aload_0
         1: arraylength
         2: ifne          14
         5: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
         8: ldc           #4                  // String help: java Fibonacci {Number}
        10: invokevirtual #5                  // Method java/io/PrintStream.print:(Ljava/lang/String;)V
        13: return
        14: aload_0
        15: iconst_0
        16: aaload
        17: invokestatic  #6                  // Method java/lang/Integer.parseInt:(Ljava/lang/String;)I
        20: istore_1
        21: invokestatic  #7                  // Method java/lang/System.currentTimeMillis:()J
        24: lstore_2
        25: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
        28: ldc           #8                  // String Fibonacci from 1 to %s:\n
        30: iconst_1
        31: anewarray     #9                  // class java/lang/Object
        34: dup
        35: iconst_0
        36: iload_1
        37: invokestatic  #10                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
        40: aastore
        41: invokevirtual #11                 // Method java/io/PrintStream.format:(Ljava/lang/String;[Ljava/lang/Object;)Ljava/io/PrintStream;
        44: pop
        45: iconst_1
        46: istore        4
        48: iload         4
        50: iload_1
        51: if_icmpgt     94
        54: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
        57: new           #12                 // class java/lang/StringBuilder
        60: dup
        61: invokespecial #13                 // Method java/lang/StringBuilder."<init>":()V
        64: iload         4
        66: invokevirtual #14                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
        69: ldc           #15                 // String :
        71: invokevirtual #16                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        74: iload         4
        76: invokestatic  #2                  // Method fib:(I)J
        79: invokevirtual #17                 // Method java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder;
        82: invokevirtual #18                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        85: invokevirtual #19                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        88: iinc          4, 1
        91: goto          48
        94: invokestatic  #7                  // Method java/lang/System.currentTimeMillis:()J
        97: lstore        4
        99: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
       102: new           #12                 // class java/lang/StringBuilder
       105: dup
       106: invokespecial #13                 // Method java/lang/StringBuilder."<init>":()V
       109: ldc           #20                 // String time:
       111: invokevirtual #16                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
       114: lload         4
       116: lload_2
       117: lsub
       118: invokevirtual #17                 // Method java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder;
       121: ldc           #21                 // String ms
       123: invokevirtual #16                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
       126: invokevirtual #18                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
       129: invokevirtual #19                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       132: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
       135: ldc           #22                 // String done.
       137: invokevirtual #19                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       140: return
      LineNumberTable:
        line 10: 0
        line 11: 5
        line 12: 13
        line 15: 14
        line 17: 21
        line 18: 25
        line 19: 45
        line 20: 54
        line 19: 88
        line 22: 94
        line 23: 99
        line 25: 132
        line 26: 140
      StackMapTable: number_of_entries = 3
        frame_type = 14 /* same */
        frame_type = 254 /* append */
          offset_delta = 33
          locals = [ int, long, int ]
        frame_type = 250 /* chop */
          offset_delta = 45
}
SourceFile: "Main.java"

那么，首先，我们看到的是对于java文件信息的一些标识、本文件的版本信息等信息，这些不是很重要，所以省略。 我们重点来说说该处的常量池。

#1 = Methodref          #9.#35         // java/lang/Object."<init>":()V

#1主要表示的是常量池的程序计数器（从1开始的），Methodref 表示的是用于记录方法信息 从#9 计数器进入 #35处被调用。而下面的

{
  public Main();
    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 1: 0
...

表示的是类名、方法名，descriptor用来区分是类还是方法，flags 来表示方法是什么样的类型，code则是代码段， LineNumberTable 表示的是这个方法在原java的第几行。 那么我们着重分析一下

         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return

上面的三条指令：

aload_0:这个操作码是aload格式操作码中的一个。它们用来把对象引用加载到操作码栈。 表示正在被访问的局部变量数组的位置，但只能是0、1、2、3 中的一个。还有一些其它类似的操作码用来载入非对象引用的数据，如iload, lload, float 和 dload。其中 i 表示 int，l 表示 long，f 表示 float，d 表示 double。局部变量数组位置大于 3 的局部变量可以用 iload, lload, float, dload 和 aload 载入。这些操作码都只需要一个操作数，即数组中的位置。 invokespecial:这些操作码属于一组函数调用的操作码，包括：invokedynamic、invokeinterface、invokespecial、invokestatic、invokevirtual。在这个 class 文件中，invokespecial 和 invokevirutal 两个指令都用到了，两者的区别是，invokevirutal 指令调用一个对象的实例方法，invokespecial 指令调用实例初始化方法、私有方法、父类方法。 return：这个操作码属于ireturn、lreturn、freturn、dreturn、areturn 和 return 操作码组。每个操作码返回一种类型的返回值，其中 i 表示 int，l 表示 long，f 表示 float，d 表示 double，a 表示 对象引用。没有前缀类型字母的 return 表示返回 void。 对于上述的指令，都会有操作数栈来进行配合。如果操作数没用完则return ，则等待系统进行垃圾回收。若用完未取到则会跑出异常。 详细的指令信息可以在：https://docs.oracle.com/javase/specs/jvms/se7/html/index.html 中见到

通过上述对于class类的简要分析，我们对jvm class文件有了一定的了解。接下来将讲述详细的内容以及如何去应用。

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浅谈 node-jvm(2) | MartinDelophy
MartinDelophy

浅谈 node-jvm(2)

2018-5-15 by MartinDelophy

上回，我们很简单的对node-jvm进行了一定的分析，但问题还是停留在表面，仅仅只是介绍了node 里面的一些特点，和java并无很多的关系，今天让我们接着来讲后续的内容。 要想很清晰的知道后续的内容，我们首先要对java class类文件做一定的了解。 我想在网上这样的文章算是比较多了，我推荐几个链接可以去看看 1.https://blog.csdn.net/u010425776/article/details/51245055 2.https://www.cnblogs.com/Qian123/p/5707562.html 3.https://blog.csdn.net/u013309870/article/details/72975536 4.http://www.blogjava.net/DLevin/archive/2011/09/05/358033.html 5.http://www.blogjava.net/DLevin/archive/2011/09/05/358034.html

看了上面几个文章之后，我们开始实战来分析一下下面的代码 首先看到的是用来针对fib 进行计算的java文件

Main.java

public class Main {

    public static long fib(int n) {
        if (n <= 1)
            return n;
        return fib(n - 1) + fib(n - 2);
    }

    public static void main(String[] args) {
        if (args.length == 0) {
            System.out.print("help: java Fibonacci {Number}");
            return;
        }

        int N = Integer.parseInt(args[0]);

        long start = System.currentTimeMillis();
        System.out.format("Fibonacci from 1 to %s:\n", N);
        for (int i = 1; i <= N; i++) {
            System.out.println(i + ": " + fib(i));
        }
        long stop = System.currentTimeMillis();
        System.out.println("time: " + (stop - start) + "ms");

        System.out.println("done.");
    }
}

该部分代码，主要是实现了最基本的fib 功能，并且调用。

那么接下来我用make 生成Main.class文件,然后通过 javap -v -verbose Main 分析一下改该类文件中的内容，内容如下：

Last modified May 7, 2018; size 1286 bytes
  MD5 checksum 115352bb394b31575a6288f7bdf82f35
  Compiled from "Main.java"
public class Main
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #9.#35         // java/lang/Object."<init>":()V
   #2 = Methodref          #23.#36        // Main.fib:(I)J
   #3 = Fieldref           #37.#38        // java/lang/System.out:Ljava/io/PrintStream;
   #4 = String             #39            // help: java Fibonacci {Number}
   #5 = Methodref          #40.#41        // java/io/PrintStream.print:(Ljava/lang/String;)V
   #6 = Methodref          #42.#43        // java/lang/Integer.parseInt:(Ljava/lang/String;)I
   #7 = Methodref          #37.#44        // java/lang/System.currentTimeMillis:()J
   #8 = String             #45            // Fibonacci from 1 to %s:\n
   #9 = Class              #46            // java/lang/Object
  #10 = Methodref          #42.#47        // java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
  #11 = Methodref          #40.#48        // java/io/PrintStream.format:(Ljava/lang/String;[Ljava/lang/Object;)Ljava/io/PrintStream;
  #12 = Class              #49            // java/lang/StringBuilder
  #13 = Methodref          #12.#35        // java/lang/StringBuilder."<init>":()V
  #14 = Methodref          #12.#50        // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
  #15 = String             #51            // :
  #16 = Methodref          #12.#52        // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #17 = Methodref          #12.#53        // java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder;
  #18 = Methodref          #12.#54        // java/lang/StringBuilder.toString:()Ljava/lang/String;
  #19 = Methodref          #40.#55        // java/io/PrintStream.println:(Ljava/lang/String;)V
  #20 = String             #56            // time:
  #21 = String             #57            // ms
  #22 = String             #58            // done.
  #23 = Class              #59            // Main
  #24 = Utf8               <init>
  #25 = Utf8               ()V
  #26 = Utf8               Code
  #27 = Utf8               LineNumberTable
  #28 = Utf8               fib
  #29 = Utf8               (I)J
  #30 = Utf8               StackMapTable
  #31 = Utf8               main
  #32 = Utf8               ([Ljava/lang/String;)V
  #33 = Utf8               SourceFile
  #34 = Utf8               Main.java
  #35 = NameAndType        #24:#25        // "<init>":()V
  #36 = NameAndType        #28:#29        // fib:(I)J
  #37 = Class              #60            // java/lang/System
  #38 = NameAndType        #61:#62        // out:Ljava/io/PrintStream;
  #39 = Utf8               help: java Fibonacci {Number}
  #40 = Class              #63            // java/io/PrintStream
  #41 = NameAndType        #64:#65        // print:(Ljava/lang/String;)V
  #42 = Class              #66            // java/lang/Integer
  #43 = NameAndType        #67:#68        // parseInt:(Ljava/lang/String;)I
  #44 = NameAndType        #69:#70        // currentTimeMillis:()J
  #45 = Utf8               Fibonacci from 1 to %s:\n
  #46 = Utf8               java/lang/Object
  #47 = NameAndType        #71:#72        // valueOf:(I)Ljava/lang/Integer;
  #48 = NameAndType        #73:#74        // format:(Ljava/lang/String;[Ljava/lang/Object;)Ljava/io/PrintStream;
  #49 = Utf8               java/lang/StringBuilder
  #50 = NameAndType        #75:#76        // append:(I)Ljava/lang/StringBuilder;
  #51 = Utf8               :
  #52 = NameAndType        #75:#77        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #53 = NameAndType        #75:#78        // append:(J)Ljava/lang/StringBuilder;
  #54 = NameAndType        #79:#80        // toString:()Ljava/lang/String;
  #55 = NameAndType        #81:#65        // println:(Ljava/lang/String;)V
  #56 = Utf8               time:
  #57 = Utf8               ms
  #58 = Utf8               done.
  #59 = Utf8               Main
  #60 = Utf8               java/lang/System
  #61 = Utf8               out
  #62 = Utf8               Ljava/io/PrintStream;
  #63 = Utf8               java/io/PrintStream
  #64 = Utf8               print
  #65 = Utf8               (Ljava/lang/String;)V
  #66 = Utf8               java/lang/Integer
  #67 = Utf8               parseInt
  #68 = Utf8               (Ljava/lang/String;)I
  #69 = Utf8               currentTimeMillis
  #70 = Utf8               ()J
  #71 = Utf8               valueOf
  #72 = Utf8               (I)Ljava/lang/Integer;
  #73 = Utf8               format
  #74 = Utf8               (Ljava/lang/String;[Ljava/lang/Object;)Ljava/io/PrintStream;
  #75 = Utf8               append
  #76 = Utf8               (I)Ljava/lang/StringBuilder;
  #77 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  #78 = Utf8               (J)Ljava/lang/StringBuilder;
  #79 = Utf8               toString
  #80 = Utf8               ()Ljava/lang/String;
  #81 = Utf8               println
{
  public Main();
    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 1: 0

  public static long fib(int);
    descriptor: (I)J
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=4, locals=1, args_size=1
         0: iload_0
         1: iconst_1
         2: if_icmpgt     8
         5: iload_0
         6: i2l
         7: lreturn
         8: iload_0
         9: iconst_1
        10: isub
        11: invokestatic  #2                  // Method fib:(I)J
        14: iload_0
        15: iconst_2
        16: isub
        17: invokestatic  #2                  // Method fib:(I)J
        20: ladd
        21: lreturn
      LineNumberTable:
        line 4: 0
        line 5: 5
        line 6: 8
      StackMapTable: number_of_entries = 1
        frame_type = 8 /* same */

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=6, locals=6, args_size=1
         0: aload_0
         1: arraylength
         2: ifne          14
         5: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
         8: ldc           #4                  // String help: java Fibonacci {Number}
        10: invokevirtual #5                  // Method java/io/PrintStream.print:(Ljava/lang/String;)V
        13: return
        14: aload_0
        15: iconst_0
        16: aaload
        17: invokestatic  #6                  // Method java/lang/Integer.parseInt:(Ljava/lang/String;)I
        20: istore_1
        21: invokestatic  #7                  // Method java/lang/System.currentTimeMillis:()J
        24: lstore_2
        25: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
        28: ldc           #8                  // String Fibonacci from 1 to %s:\n
        30: iconst_1
        31: anewarray     #9                  // class java/lang/Object
        34: dup
        35: iconst_0
        36: iload_1
        37: invokestatic  #10                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
        40: aastore
        41: invokevirtual #11                 // Method java/io/PrintStream.format:(Ljava/lang/String;[Ljava/lang/Object;)Ljava/io/PrintStream;
        44: pop
        45: iconst_1
        46: istore        4
        48: iload         4
        50: iload_1
        51: if_icmpgt     94
        54: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
        57: new           #12                 // class java/lang/StringBuilder
        60: dup
        61: invokespecial #13                 // Method java/lang/StringBuilder."<init>":()V
        64: iload         4
        66: invokevirtual #14                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
        69: ldc           #15                 // String :
        71: invokevirtual #16                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        74: iload         4
        76: invokestatic  #2                  // Method fib:(I)J
        79: invokevirtual #17                 // Method java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder;
        82: invokevirtual #18                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        85: invokevirtual #19                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        88: iinc          4, 1
        91: goto          48
        94: invokestatic  #7                  // Method java/lang/System.currentTimeMillis:()J
        97: lstore        4
        99: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
       102: new           #12                 // class java/lang/StringBuilder
       105: dup
       106: invokespecial #13                 // Method java/lang/StringBuilder."<init>":()V
       109: ldc           #20                 // String time:
       111: invokevirtual #16                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
       114: lload         4
       116: lload_2
       117: lsub
       118: invokevirtual #17                 // Method java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder;
       121: ldc           #21                 // String ms
       123: invokevirtual #16                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
       126: invokevirtual #18                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
       129: invokevirtual #19                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       132: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
       135: ldc           #22                 // String done.
       137: invokevirtual #19                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       140: return
      LineNumberTable:
        line 10: 0
        line 11: 5
        line 12: 13
        line 15: 14
        line 17: 21
        line 18: 25
        line 19: 45
        line 20: 54
        line 19: 88
        line 22: 94
        line 23: 99
        line 25: 132
        line 26: 140
      StackMapTable: number_of_entries = 3
        frame_type = 14 /* same */
        frame_type = 254 /* append */
          offset_delta = 33
          locals = [ int, long, int ]
        frame_type = 250 /* chop */
          offset_delta = 45
}
SourceFile: "Main.java"

那么，首先，我们看到的是对于java文件信息的一些标识、本文件的版本信息等信息，这些不是很重要，所以省略。 我们重点来说说该处的常量池。

#1 = Methodref          #9.#35         // java/lang/Object."<init>":()V

#1主要表示的是常量池的程序计数器（从1开始的），Methodref 表示的是用于记录方法信息 从#9 计数器进入 #35处被调用。而下面的

{
  public Main();
    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 1: 0
...

表示的是类名、方法名，descriptor用来区分是类还是方法，flags 来表示方法是什么样的类型，code则是代码段， LineNumberTable 表示的是这个方法在原java的第几行。 那么我们着重分析一下

         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return

上面的三条指令：

aload_0:这个操作码是aload格式操作码中的一个。它们用来把对象引用加载到操作码栈。 表示正在被访问的局部变量数组的位置，但只能是0、1、2、3 中的一个。还有一些其它类似的操作码用来载入非对象引用的数据，如iload, lload, float 和 dload。其中 i 表示 int，l 表示 long，f 表示 float，d 表示 double。局部变量数组位置大于 3 的局部变量可以用 iload, lload, float, dload 和 aload 载入。这些操作码都只需要一个操作数，即数组中的位置。 invokespecial:这些操作码属于一组函数调用的操作码，包括：invokedynamic、invokeinterface、invokespecial、invokestatic、invokevirtual。在这个 class 文件中，invokespecial 和 invokevirutal 两个指令都用到了，两者的区别是，invokevirutal 指令调用一个对象的实例方法，invokespecial 指令调用实例初始化方法、私有方法、父类方法。 return：这个操作码属于ireturn、lreturn、freturn、dreturn、areturn 和 return 操作码组。每个操作码返回一种类型的返回值，其中 i 表示 int，l 表示 long，f 表示 float，d 表示 double，a 表示 对象引用。没有前缀类型字母的 return 表示返回 void。 对于上述的指令，都会有操作数栈来进行配合。如果操作数没用完则return ，则等待系统进行垃圾回收。若用完未取到则会跑出异常。 详细的指令信息可以在：https://docs.oracle.com/javase/specs/jvms/se7/html/index.html 中见到

通过上述对于class类的简要分析，我们对jvm class文件有了一定的了解。接下来将讲述详细的内容以及如何去应用。

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