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构造方法与方法调用指令分析 || JVM类加载与字节码技术
静态代码块
源码
package cn.knightzz.jvm.bytecode; /** * @author 王天赐 * @title: StaticCode * @projectName hm-jvm-codes * @description: 静态代码块 * @website http://knightzz.cn/ * @github https://github.com/knightzz1998 * @create: 2022-10-19 19:42 */ public class StaticCode { static { i = 30; } static int i = 10; static { i = 20; } }- 1
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字节码
$ javap -v StaticCode.class Classfile /F:/JavaCode/hm-jvm-codes/jvm-chapter-03/target/classes/cn/knightzz/jvm/bytecode/StaticCode.class Last modified 2022-10-19; size 408 bytes MD5 checksum d8dc4cae931bd2602bdb99bd72e511ce Compiled from "StaticCode.java" public class cn.knightzz.jvm.bytecode.StaticCode minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #4.#17 // java/lang/Object."":()V #2 = Fieldref #3.#18 // cn/knightzz/jvm/bytecode/StaticCode.i:I #3 = Class #19 // cn/knightzz/jvm/bytecode/StaticCode #4 = Class #20 // java/lang/Object #5 = Utf8 i #6 = Utf8 I #7 = Utf8 <init> #8 = Utf8 ()V #9 = Utf8 Code #10 = Utf8 LineNumberTable #11 = Utf8 LocalVariableTable #12 = Utf8 this #13 = Utf8 Lcn/knightzz/jvm/bytecode/StaticCode; #14 = Utf8 <clinit> #15 = Utf8 SourceFile #16 = Utf8 StaticCode.java #17 = NameAndType #7:#8 // "":()V #18 = NameAndType #5:#6 // i:I #19 = Utf8 cn/knightzz/jvm/bytecode/StaticCode #20 = Utf8 java/lang/Object { static int i; descriptor: I flags: ACC_STATIC public cn.knightzz.jvm.bytecode.StaticCode(); descriptor: ()V flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: invokespecial #1 // Method java/lang/Object."":()V 4: return LineNumberTable: line 12: 0 LocalVariableTable: Start Length Slot Name Signature 0 5 0 this Lcn/knightzz/jvm/bytecode/StaticCode; static {}; descriptor: ()V flags: ACC_STATIC Code: stack=1, locals=0, args_size=0 0: bipush 10 2: putstatic #2 // Field i:I 5: bipush 20 7: putstatic #2 // Field i:I 10: bipush 30 12: putstatic #2 // Field i:I 15: return LineNumberTable: line 14: 0 line 17: 5 line 21: 10 line 22: 15 } SourceFile: "StaticCode.java"- 1
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编译器会按从上至下的顺序，收集所有 static 静态代码块和静态成员赋值的代码，合并为一个特殊的方法
()V static {}; descriptor: ()V flags: ACC_STATIC Code: stack=1, locals=0, args_size=0 0: bipush 10 2: putstatic #2 // Field i:I 5: bipush 20 7: putstatic #2 // Field i:I 10: bipush 30 12: putstatic #2 // Field i:I 15: return- 1
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()V 改变代码位置
package cn.knightzz.jvm.bytecode; /** * @author 王天赐 * @title: StaticCode * @projectName hm-jvm-codes * @description: 静态代码块 * @website http://knightzz.cn/ * @github https://github.com/knightzz1998 * @create: 2022-10-19 19:42 */ public class StaticCode { static { i = 30; } static int i = 10; static { i = 20; } }- 1
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原本的字节码
0: bipush 10 2: putstatic #2 // Field i:I 5: bipush 20 7: putstatic #2 // Field i:I 10: bipush 30 12: putstatic #2 // Field i:I 15: return- 1
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改变位置后的字节码 :
static {}; descriptor: ()V flags: ACC_STATIC Code: stack=1, locals=0, args_size=0 0: bipush 30 2: putstatic #2 // Field i:I 5: bipush 10 7: putstatic #2 // Field i:I 10: bipush 20 12: putstatic #2 // Field i:I 15: return- 1
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初始化方法
初始化方法 :
()V 源码
package cn.knightzz.jvm.bytecode; /** * @author 王天赐 * @title: InitCode * @projectName hm-jvm-codes * @description: * @website http://knightzz.cn/ * @github https://github.com/knightzz1998 * @create: 2022-10-19 20:01 */ public class InitCode { private String a = "s1"; { b = 20; } private int b = 10; { a = "s2"; } public InitCode(String a, int b) { this.a = a; this.b = b; } public static void main(String[] args) { InitCode d = new InitCode("s3", 30); System.out.println(d.a); System.out.println(d.b); } }- 1
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字节码
$ javap -v InitCode.class Classfile /F:/JavaCode/hm-jvm-codes/jvm-chapter-03/target/classes/cn/knightzz/jvm/bytecode/InitCode.class Last modified 2022-10-19; size 833 bytes MD5 checksum 91c75c2ea803f91f8b1244a3921a4fda Compiled from "InitCode.java" public class cn.knightzz.jvm.bytecode.InitCode minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #12.#31 // java/lang/Object."":()V #2 = String #32 // s1 #3 = Fieldref #6.#33 // cn/knightzz/jvm/bytecode/InitCode.a:Ljava/lang/String; #4 = Fieldref #6.#34 // cn/knightzz/jvm/bytecode/InitCode.b:I #5 = String #35 // s2 #6 = Class #36 // cn/knightzz/jvm/bytecode/InitCode #7 = String #37 // s3 #8 = Methodref #6.#38 // cn/knightzz/jvm/bytecode/InitCode."":(Ljava/lang/String;I)V #9 = Fieldref #39.#40 // java/lang/System.out:Ljava/io/PrintStream; #10 = Methodref #41.#42 // java/io/PrintStream.println:(Ljava/lang/String;)V #11 = Methodref #41.#43 // java/io/PrintStream.println:(I)V #12 = Class #44 // java/lang/Object #13 = Utf8 a #14 = Utf8 Ljava/lang/String; #15 = Utf8 b #16 = Utf8 I #17 = Utf8 <init> #18 = Utf8 (Ljava/lang/String;I)V #19 = Utf8 Code #20 = Utf8 LineNumberTable #21 = Utf8 LocalVariableTable #22 = Utf8 this #23 = Utf8 Lcn/knightzz/jvm/bytecode/InitCode; #24 = Utf8 main #25 = Utf8 ([Ljava/lang/String;)V #26 = Utf8 args #27 = Utf8 [Ljava/lang/String; #28 = Utf8 d #29 = Utf8 SourceFile #30 = Utf8 InitCode.java #31 = NameAndType #17:#45 // "":()V #32 = Utf8 s1 #33 = NameAndType #13:#14 // a:Ljava/lang/String; #34 = NameAndType #15:#16 // b:I #35 = Utf8 s2 #36 = Utf8 cn/knightzz/jvm/bytecode/InitCode #37 = Utf8 s3 #38 = NameAndType #17:#18 // "":(Ljava/lang/String;I)V #39 = Class #46 // java/lang/System #40 = NameAndType #47:#48 // out:Ljava/io/PrintStream; #41 = Class #49 // java/io/PrintStream #42 = NameAndType #50:#51 // println:(Ljava/lang/String;)V #43 = NameAndType #50:#52 // println:(I)V #44 = Utf8 java/lang/Object #45 = Utf8 ()V #46 = Utf8 java/lang/System #47 = Utf8 out #48 = Utf8 Ljava/io/PrintStream; #49 = Utf8 java/io/PrintStream #50 = Utf8 println #51 = Utf8 (Ljava/lang/String;)V #52 = Utf8 (I)V { public cn.knightzz.jvm.bytecode.InitCode(java.lang.String, int); descriptor: (Ljava/lang/String;I)V flags: ACC_PUBLIC Code: stack=2, locals=3, args_size=3 0: aload_0 1: invokespecial #1 // Method java/lang/Object."":()V 4: aload_0 5: ldc #2 // String s1 7: putfield #3 // Field a:Ljava/lang/String; 10: aload_0 11: bipush 20 13: putfield #4 // Field b:I 16: aload_0 17: bipush 10 19: putfield #4 // Field b:I 22: aload_0 23: ldc #5 // String s2 25: putfield #3 // Field a:Ljava/lang/String; 28: aload_0 29: aload_1 30: putfield #3 // Field a:Ljava/lang/String; 33: aload_0 34: iload_2 35: putfield #4 // Field b:I 38: return LineNumberTable: line 26: 0 line 14: 4 line 17: 10 line 20: 16 line 23: 22 line 27: 28 line 28: 33 line 29: 38 LocalVariableTable: Start Length Slot Name Signature 0 39 0 this Lcn/knightzz/jvm/bytecode/InitCode; 0 39 1 a Ljava/lang/String; 0 39 2 b I public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=4, locals=2, args_size=1 0: new #6 // class cn/knightzz/jvm/bytecode/InitCode 3: dup 4: ldc #7 // String s3 6: bipush 30 8: invokespecial #8 // Method "":(Ljava/lang/String;I)V 11: astore_1 12: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream; 15: aload_1 16: getfield #3 // Field a:Ljava/lang/String; 19: invokevirtual #10 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 22: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream; 25: aload_1 26: getfield #4 // Field b:I 29: invokevirtual #11 // Method java/io/PrintStream.println:(I)V 32: return LineNumberTable: line 32: 0 line 33: 12 line 34: 22 line 35: 32 LocalVariableTable: Start Length Slot Name Signature 0 33 0 args [Ljava/lang/String; 12 21 1 d Lcn/knightzz/jvm/bytecode/InitCode; } SourceFile: "InitCode.java"- 1
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编译器会按从上至下的顺序，收集所有 {} 代码块和成员变量赋值的代码，形成新的构造方法，但原始构造方法内的代码总是在最后
public cn.knightzz.jvm.bytecode.InitCode(java.lang.String, int); descriptor: (Ljava/lang/String;I)V flags: ACC_PUBLIC Code: stack=2, locals=3, args_size=3 0: aload_0 1: invokespecial #1 // Method java/lang/Object."":()V 4: aload_0 // this // ---------- private String a = "s1"; --------------------- 5: ldc #2 // String "s1" 将 s1 变量加载到操作数栈 7: putfield #3 // Field a:Ljava/lang/String; ==> this.a = s1 10: aload_0 // this 11: bipush 20 13: putfield #4 // Field b:I 16: aload_0 // ----------- private int b = 10; -------------------------- 17: bipush 10 // ---------- a = "s1"; --------------------- 19: putfield #4 // Field b:I 22: aload_0 // ---------- a = "s1"; --------------------- 23: ldc #5 // String s2 // 25: putfield #3 // Field a:Ljava/lang/String; ==> this.a 28: aload_0 29: aload_1 30: putfield #3 // Field a:Ljava/lang/String; 33: aload_0 34: iload_2 35: putfield #4 // Field b:I ==> this.b 38: return- 1
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- putfield 设置对象中字段的值
- ldc #2 将 “s1” 变量加载到操作数栈
- putfield #3 : 将 “s1” 赋值给操作数a
方法调用
看一下几种不同的方法调用对应的字节码指令
源码
package cn.knightzz.jvm.bytecode; /** * @author 王天赐 * @title: MethodInvocation * @projectName hm-jvm-codes * @description: 方法调用 * @website http://knightzz.cn/ * @github https://github.com/knightzz1998 * @create: 2022-10-19 20:32 */ public class MethodInvocation { public MethodInvocation() { } private void test1() { } private final void test2() { } public void test3() { } public static void test4() { } public static void main(String[] args) { MethodInvocation d = new MethodInvocation(); d.test1(); d.test2(); d.test3(); d.test4(); MethodInvocation.test4(); } }- 1
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字节码
$ javap -v MethodInvocation.class Classfile /F:/JavaCode/hm-jvm-codes/jvm-chapter-03/target/classes/cn/knightzz/jvm/bytecode/MethodInvocation.class Last modified 2022-10-19; size 798 bytes MD5 checksum e3ab39988b1374e15c9c4019127d11d0 Compiled from "MethodInvocation.java" public class cn.knightzz.jvm.bytecode.MethodInvocation minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #8.#27 // java/lang/Object."":()V #2 = Class #28 // cn/knightzz/jvm/bytecode/MethodInvocation #3 = Methodref #2.#27 // cn/knightzz/jvm/bytecode/MethodInvocation."":()V #4 = Methodref #2.#29 // cn/knightzz/jvm/bytecode/MethodInvocation.test1:()V #5 = Methodref #2.#30 // cn/knightzz/jvm/bytecode/MethodInvocation.test2:()V #6 = Methodref #2.#31 // cn/knightzz/jvm/bytecode/MethodInvocation.test3:()V #7 = Methodref #2.#32 // cn/knightzz/jvm/bytecode/MethodInvocation.test4:()V #8 = Class #33 // java/lang/Object #9 = Utf8 <init> #10 = Utf8 ()V #11 = Utf8 Code #12 = Utf8 LineNumberTable #13 = Utf8 LocalVariableTable #14 = Utf8 this #15 = Utf8 Lcn/knightzz/jvm/bytecode/MethodInvocation; #16 = Utf8 test1 #17 = Utf8 test2 #18 = Utf8 test3 #19 = Utf8 test4 #20 = Utf8 main #21 = Utf8 ([Ljava/lang/String;)V #22 = Utf8 args #23 = Utf8 [Ljava/lang/String; #24 = Utf8 d #25 = Utf8 SourceFile #26 = Utf8 MethodInvocation.java #27 = NameAndType #9:#10 // "":()V #28 = Utf8 cn/knightzz/jvm/bytecode/MethodInvocation #29 = NameAndType #16:#10 // test1:()V #30 = NameAndType #17:#10 // test2:()V #31 = NameAndType #18:#10 // test3:()V #32 = NameAndType #19:#10 // test4:()V #33 = Utf8 java/lang/Object { public cn.knightzz.jvm.bytecode.MethodInvocation(); descriptor: ()V flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: invokespecial #1 // Method java/lang/Object."":()V 4: return LineNumberTable: line 14: 0 line 15: 4 LocalVariableTable: Start Length Slot Name Signature 0 5 0 this Lcn/knightzz/jvm/bytecode/MethodInvocation; public void test3(); descriptor: ()V flags: ACC_PUBLIC Code: stack=0, locals=1, args_size=1 0: return LineNumberTable: line 24: 0 LocalVariableTable: Start Length Slot Name Signature 0 1 0 this Lcn/knightzz/jvm/bytecode/MethodInvocation; public static void test4(); descriptor: ()V flags: ACC_PUBLIC, ACC_STATIC Code: stack=0, locals=0, args_size=0 0: return LineNumberTable: line 27: 0 public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=2, locals=2, args_size=1 0: new #2 // class cn/knightzz/jvm/bytecode/MethodInvocation 3: dup 4: invokespecial #3 // Method "":()V 7: astore_1 8: aload_1 9: invokespecial #4 // Method test1:()V 12: aload_1 13: invokespecial #5 // Method test2:()V 16: aload_1 17: invokevirtual #6 // Method test3:()V 20: aload_1 21: pop 22: invokestatic #7 // Method test4:()V 25: invokestatic #7 // Method test4:()V 28: return LineNumberTable: line 30: 0 line 31: 8 line 32: 12 line 33: 16 line 34: 20 line 35: 25 line 36: 28 LocalVariableTable: Start Length Slot Name Signature 0 29 0 args [Ljava/lang/String; 8 21 1 d Lcn/knightzz/jvm/bytecode/MethodInvocation; } SourceFile: "MethodInvocation.java"- 1
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Main 函数中的调用字节码 :
stack=2, locals=2, args_size=1 0: new #2 // class cn/knightzz/jvm/bytecode/MethodInvocation 3: dup 4: invokespecial #3 // Method "":()V 7: astore_1 8: aload_1 9: invokespecial #4 // Method test1:()V 12: aload_1 13: invokespecial #5 // Method test2:()V 16: aload_1 17: invokevirtual #6 // Method test3:()V 20: aload_1 // 将对象引用放入操作数栈 21: pop // 但是静态方法不需要对象, 所以又把它从操作数栈移除了 22: invokestatic #7 // Method test4:()V 25: invokestatic #7 // Method test4:()V 28: return- 1
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new #2 : 是创建【对象】，给对象分配堆内存，执行成功会将【对象引用】压入操作数栈
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dup 将栈顶的地址再次进行复制一份到操作数栈, 本例即为【对象引用】,这样操作数栈就有了两个对象引用，
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一个是要配合 invokespecial 调用该对象的构造方法
"（会消耗掉栈顶一个引用）":()V -
另一个要配合 astore_1 赋值给局部变量 d (插槽1的变量)
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最终方法（final），私有方法（private），构造方法都是由 invokespecial 指令来调用，属于静
态绑定
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普通成员方法是由 invokevirtual 调用，属于动态绑定，即支持多态, 因为它在调用之前并不知道具体是哪个对象调用的, 可能是子类也可能是父类的
- 因为非 final 和 private 的方法是可以被重写的
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成员方法与静态方法调用的另一个区别是，执行方法前是否需要【对象引用】
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比较有意思的是 d.test4(); 是通过【对象引用】调用一个静态方法
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可以看到在调用invokestatic 之前执行了 pop 指令，把【对象引用】从操作数栈弹掉了
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还有一个执行 invokespecial 的情况是通过 super 调用父类方法
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- 原文地址：https://blog.csdn.net/weixin_40040107/article/details/127415751
