JVM-JAVA基本类型

 1 package javap.fload;
 2 
 3 import static jdk.nashorn.internal.objects.Global.Infinity;
 4 
 5 public class FloatTest {
 6     public static void main(String[] args) {
 7         System.out.println(+0.0F == -0.0F);
 8         System.out.println(Float.intBitsToFloat(0x8000000) == -0.0F);
 9 
10         float res = (float) (1.2 / -0.0F);
11         System.out.println(1.2 / -0.0F);  // -Infinity负无穷，任意正浮点数除以 -0.0F 得到的值
12         System.out.println(1.3 / 0.0F);  // Infinity正无穷，任意正浮点数（不包括 +0.0F）除以 +0.0F 得到的值
13         System.out.println(0.0F / 0.0F);  // Not-a-Number
14 
15         /**
16          * 正无穷+1，还是等于正无穷
17          */
18         System.out.println("Infinity+1=" + (Infinity+1));
19 
20 
21         /**
22          * NaN 有一个有趣的特性：除了“!=”始终返回 true 之外，所有其他比较结果都会返回 false。
23          * 例来说，“NaN<1.0F”返回 false，而“NaN>=1.0F”同样返回 false。
24          * 对于任意浮点数 f，不管它是 0 还是 NaN，“f!=NaN”始终会返回 true，而“f==NaN”始终会返回 false。
25          */
26         // 因此，我们在程序里做浮点数比较的时候，需要考虑上述特性。在本专栏的第二部分，我会介绍这个特性给向量化比较带来什么麻烦
27         float f = 1.0F;
28         if (f != Float.NaN) { // 返回true，其余比较返回false
29             System.out.println("f != Float.NaN");
30         }
31         if (f < Float.NaN) {
32             System.out.println("f < Float.NaN");
33         }
34 
35     }
36 }

结果：

true
false
-Infinity
Infinity
NaN
Infinity+1=Infinity
Infinity
f != Float.NaN

asmtools.jar的使用方法：https://wiki.openjdk.java.net/display/CodeTools/Chapter+2

OpenJDK里的AsmTools简介 http://hengyunabc.github.io/openjdk-asmtools/  （修改.class字节码的工具）

 1 MacBook-Pro-2:javap$ java -jar asmtools.jar
 2 Usage: 
 3 to run an assembly tool: 
 4    $ java -jar asmtools.jar toolName [args...] 
 5    where toolName one of: jasm, jdis, jcoder, jdec, jcdec 
 6 to get the version: 
 7    $ java -jar asmtools.jar -version 
 8 to get this message 
 9    $ java -jar asmtools.jar -?|-h|-help
10 
11 No arguments provided!  See options above.

代码：（静态变量修改）

 1 package javap.fload;
 2 
 3 public class FooBoolean {
 4     static boolean boolValue;
 5 
 6     public static void main(String[] args) {
 7         boolValue = true; // 将这个true替换为2或者3，再看看打印结果
 8         if (boolValue) System.out.println("Hello, Java!");
 9         if (boolValue == true) System.out.println("Hello, JVM!");
10     }
11 }

javap -p -v FooBoolean.class结果：

MacBook-Pro-2:fload$ javap -p -v FooBoolean.class 
Classfile /Users/wuxiong.wx/work/code/testDemo/src/main/java/javap/fload/FooBoolean.class
  Last modified 2020-11-16; size 668 bytes
  MD5 checksum 2693d7088f23a5890569763240da4915
  Compiled from "FooBoolean.java"
public class javap.fload.FooBoolean
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #8.#25         // java/lang/Object."<init>":()V
   #2 = Fieldref           #7.#26         // javap/fload/FooBoolean.boolValue:Z
   #3 = Fieldref           #27.#28        // java/lang/System.out:Ljava/io/PrintStream;
   #4 = String             #29            // Hello, Java!
   #5 = Methodref          #30.#31        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #6 = String             #32            // Hello, JVM!
   #7 = Class              #33            // javap/fload/FooBoolean
   #8 = Class              #34            // java/lang/Object
   #9 = Utf8               boolValue
  #10 = Utf8               Z
  #11 = Utf8               <init>
  #12 = Utf8               ()V
  #13 = Utf8               Code
  #14 = Utf8               LineNumberTable
  #15 = Utf8               LocalVariableTable
  #16 = Utf8               this
  #17 = Utf8               Ljavap/fload/FooBoolean;
  #18 = Utf8               main
  #19 = Utf8               ([Ljava/lang/String;)V
  #20 = Utf8               args
  #21 = Utf8               [Ljava/lang/String;
  #22 = Utf8               StackMapTable
  #23 = Utf8               SourceFile
  #24 = Utf8               FooBoolean.java
  #25 = NameAndType        #11:#12        // "<init>":()V
  #26 = NameAndType        #9:#10         // boolValue:Z
  #27 = Class              #35            // java/lang/System
  #28 = NameAndType        #36:#37        // out:Ljava/io/PrintStream;
  #29 = Utf8               Hello, Java!
  #30 = Class              #38            // java/io/PrintStream
  #31 = NameAndType        #39:#40        // println:(Ljava/lang/String;)V
  #32 = Utf8               Hello, JVM!
  #33 = Utf8               javap/fload/FooBoolean
  #34 = Utf8               java/lang/Object
  #35 = Utf8               java/lang/System
  #36 = Utf8               out
  #37 = Utf8               Ljava/io/PrintStream;
  #38 = Utf8               java/io/PrintStream
  #39 = Utf8               println
  #40 = Utf8               (Ljava/lang/String;)V
{
  static boolean boolValue;
    descriptor: Z
    flags: ACC_STATIC

  public javap.fload.FooBoolean();
    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
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Ljavap/fload/FooBoolean;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: iconst_1
         1: putstatic     #2                  // Field boolValue:Z
         4: getstatic     #2                  // Field boolValue:Z
         7: ifeq          18
        10: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
        13: ldc           #4                  // String Hello, Java!
        15: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        18: getstatic     #2                  // Field boolValue:Z
        21: iconst_1
        22: if_icmpne     33
        25: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
        28: ldc           #6                  // String Hello, JVM!
        30: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        33: return
      LineNumberTable:
        line 7: 0
        line 8: 4
        line 9: 18
        line 10: 33
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      34     0  args   [Ljava/lang/String;
      StackMapTable: number_of_entries = 2
        frame_type = 18 /* same */
        frame_type = 14 /* same */
}
SourceFile: "FooBoolean.java"

 java -jar ../asmtools.jar jdis FooBoolean.class > FooBoolean.asm.1

 1 MacBook-Pro-2:fload$ cat FooBoolean.asm.1 
 2 package  javap/fload;
 3 
 4 super public class FooBoolean
 5         version 52:0
 6 {
 7 
 8 static Field boolValue:Z;
 9 
10 public Method "<init>":"()V"
11         stack 1 locals 1
12 {
13                 aload_0;
14                 invokespecial   Method java/lang/Object."<init>":"()V";
15                 return;
16         
17 }
18 
19 public static Method main:"([Ljava/lang/String;)V"
20         stack 2 locals 1
21 {
22                 iconst_1;         // 将这个替换为iconst_2或iconst_3，然后保存结果为FooBoolean.asm.2或FooBoolean.asm.3
23                 putstatic       Field boolValue:"Z";
24                 getstatic       Field boolValue:"Z";
25                 ifeq    L18;
26                 getstatic       Field java/lang/System.out:"Ljava/io/PrintStream;";
27                 ldc     String "Hello, Java!";
28                 invokevirtual   Method java/io/PrintStream.println:"(Ljava/lang/String;)V";
29         L18:    stack_frame_type same;
30                 getstatic       Field boolValue:"Z";
31                 iconst_1; // 不替换，这里是true的int值
32                 if_icmpne       L33;
33                 getstatic       Field java/lang/System.out:"Ljava/io/PrintStream;";
34                 ldc     String "Hello, JVM!";
35                 invokevirtual   Method java/io/PrintStream.println:"(Ljava/lang/String;)V";
36         L33:    stack_frame_type same;
37                 return;
38         
39 }
40 
41 } // end Class FooBoolean

输出结果：

$ java -jar ../asmtools.jar jasm FooBoolean.asm.1
$ java javap.fload.FooBoolean // 两行都打印输出
Hello, Java!
Hello, JVM!
$ java -jar ../asmtools.jar jasm FooBoolean.asm.2
$ java javap.fload.FooBoolean  // 没有输出
$ java -jar ../asmtools.jar jasm FooBoolean.asm.3
$ java javap.fload.FooBoolean  // 两行都打印输出
Hello, Java!
Hello, JVM!

解释：

  • 将局部变量区存到堆中（掩码）：将boolean 保存在静态域中,指定了其类型为'Z',当修改为2时取低位最后一位为0,当修改为3时取低位最后一位为1，则说明boolean的掩码处理是取低位的最后一位
  • 将堆中的数存到局部变量区（除了long、double外都将当做int型处理）： 

代码：（局部变量修改）

1 public class App{
2  public static void main(String[] args) {
3   boolean flag = true;
4   if (flag) System.out.println("Hello, Java!");
5   if (flag == true) System.out.println("Hello, JVM!");
6  }
7 }

$ java -jar asmtools.jar jdis App.class > App.asm

 1 super public class App
 2     version 52:0
 3 {
 4 
 5 
 6 public Method "<init>":"()V"
 7     stack 1 locals 1
 8 {
 9         aload_0;
10         invokespecial    Method java/lang/Object."<init>":"()V";
11         return;
12 
13 }
14 
15 public static Method main:"([Ljava/lang/String;)V"
16     stack 2 locals 2
17 {
18         iconst_1;       // 此处替换为2或3，相应文件为App.asm.2或App.asm.3
19         istore_1;
20         iload_1;
21         ifeq    L14;
22         getstatic    Field java/lang/System.out:"Ljava/io/PrintStream;";
23         ldc    String "Hello, Java!";
24         invokevirtual    Method java/io/PrintStream.println:"(Ljava/lang/String;)V";
25     L14:    stack_frame_type append;
26         locals_map int;
27         iload_1;
28         iconst_1;
29         if_icmpne    L27;
30         getstatic    Field java/lang/System.out:"Ljava/io/PrintStream;";
31         ldc    String "Hello, JVM!";
32         invokevirtual    Method java/io/PrintStream.println:"(Ljava/lang/String;)V";
33     L27:    stack_frame_type same;
34         return;
35 
36 }
37 
38 } // end Class App
 1 $ java -jar asmtools.jar jasm App.asm  
 2 $ java App  // 输出全部
 3 Hello, Java!
 4 Hello, JVM!
 5 $ java -jar asmtools.jar jasm App.asm.2
 6 $ java App  // 输出第一行
 7 Hello, Java!
 8 $ java -jar asmtools.jar jasm App.asm.3
 9 $ java App  // 输出第一行
10 Hello, Java!

 

参考文献：https://time.geekbang.org/column/article/11503

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原文地址：https://www.cnblogs.com/wxdlut/p/13985488.html