注意: javap 檢視【class檔案的位元組碼】資訊
Java位元組碼深入解析
一:Java位元組代碼的組織形式
類檔案{
OxCAFEBABE,小版本号,大版本号,常量池大小,常量池數組,通路控制标記,目前類資訊,父類資訊,實作的接口個數,實作的接口資訊數組,域個數,域資訊數組,方法個數,方法資訊數組,屬性個數,屬性資訊數組
}
二:檢視方法 --- javap指令
例子:有一個Java類Demo.java
/**
* @author honglei
* @since 2019-08-11
*/
public class Demo {
/**
* str1
*/
private String str1;
/**
* str2
*/
private String str2;
/**
* num1
*/
private int num1;
/**
* num2
*/
private int num2;
/**
* str3
*/
public static final String STATIC_DATA = "hello world";
/**
* str1
*/
private void sayHello1() {
System.out.println("this is method1...");
}
/**
* sayHello2
*/
private void sayHello2() {
System.out.println("this is method2...");
}
/**
* sayHello3
*/
private void sayHello3() {
System.out.println("this is method3...");
}
}
通過jdk自帶的反編譯工具指令 javap 可以檢視class檔案的位元組碼資訊
D:\>javap -verbose Demo >> Demo.txt |
Demo.txt:
sai:springbootdemo ws$ javap -verbose Demo.class
Classfile /Users/ws/dev/SourceTree/springboot/springboot-demo/target/classes/com/example/springbootdemo/Demo.class
Last modified Aug 11, 2019; size 1055 bytes
MD5 checksum 98bea2d49bf0c5386ac21e3cae91d404
Compiled from "Demo.java"
public class com.example.springbootdemo.Demo
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #9.#36 // java/lang/Object."<init>":()V
#2 = Fieldref #37.#38 // java/lang/System.out:Ljava/io/PrintStream;
#3 = String #39 // this is method1...
#4 = Methodref #40.#41 // java/io/PrintStream.println:(Ljava/lang/String;)V
#5 = String #42 // this is method2...
#6 = String #43 // this is method3...
#7 = String #44 // test
#8 = Class #45 // com/example/springbootdemo/Demo
#9 = Class #46 // java/lang/Object
#10 = Utf8 str1
#11 = Utf8 Ljava/lang/String;
#12 = Utf8 str2
#13 = Utf8 num1
#14 = Utf8 I
#15 = Utf8 num2
#16 = Utf8 STATIC_DATA
#17 = Utf8 ConstantValue
#18 = String #47 // hello world
#19 = Utf8 <init>
#20 = Utf8 ()V
#21 = Utf8 Code
#22 = Utf8 LineNumberTable
#23 = Utf8 LocalVariableTable
#24 = Utf8 this
#25 = Utf8 Lcom/example/springbootdemo/Demo;
#26 = Utf8 sayHello1
#27 = Utf8 sayHello2
#28 = Utf8 sayHello3
#29 = Utf8 main
#30 = Utf8 ([Ljava/lang/String;)V
#31 = Utf8 args
#32 = Utf8 [Ljava/lang/String;
#33 = Utf8 MethodParameters
#34 = Utf8 SourceFile
#35 = Utf8 Demo.java
#36 = NameAndType #19:#20 // "<init>":()V
#37 = Class #48 // java/lang/System
#38 = NameAndType #49:#50 // out:Ljava/io/PrintStream;
#39 = Utf8 this is method1...
#40 = Class #51 // java/io/PrintStream
#41 = NameAndType #52:#53 // println:(Ljava/lang/String;)V
#42 = Utf8 this is method2...
#43 = Utf8 this is method3...
#44 = Utf8 test
#45 = Utf8 com/example/springbootdemo/Demo
#46 = Utf8 java/lang/Object
#47 = Utf8 hello world
#48 = Utf8 java/lang/System
#49 = Utf8 out
#50 = Utf8 Ljava/io/PrintStream;
#51 = Utf8 java/io/PrintStream
#52 = Utf8 println
#53 = Utf8 (Ljava/lang/String;)V
{
public static final java.lang.String STATIC_DATA;
descriptor: Ljava/lang/String;
flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL
ConstantValue: String hello world
public com.example.springbootdemo.Demo();
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 7: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/example/springbootdemo/Demo;
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: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #7 // String test
5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 54: 0
line 55: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 args [Ljava/lang/String;
MethodParameters:
Name Flags
args
}
SourceFile: "Demo.java"
解析:
1、版本号 major version: 49 //java版本 jdk1.6顯示的是50, jdk1.5顯示的是49,jdk1.4顯示的是58 , 高版本能執行低版本的class檔案
2、常量池Constant pool
Method:方法
Field:字段
String:字元串
Asciz:簽名如<init>由jvm調用,其他是不能夠去調用它的
NameAndType:變量名的類型
Class:類
通過位元組碼,我們可以看到Demo類 繼承于java.lang.Object,如果類中沒有顯式聲明構造函數的話,編譯器會插入一個預設無參的構造函數(構造函數在JVM級别是顯示成<init>的普通函數)。
三:檢測代碼的效率問題
學習Java的過程中,都會了解到字元串合并時要用到StringBuffer 來代替String,那下面就來通過Java位元組碼來驗證兩種方式的效率性。
例子:一個Java類 TestString.java
|
javap –c TestString 後位元組碼資訊:
|
從上面編譯後的位元組碼資訊可以看出來,方法testString 調用了五個方法:new 、invokestatic 、invokespecial 和兩個invokevirtual ; 而testStringBuffer 方法隻調用了兩個invokevirtual 方法。第一個方法比第二個方法多做了好多工作,其效率當然是要低的。而且我們從java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
可以看出來其實對于String字元串合并,内部還是轉化為StringBuilder的方法調用,這是因為String是長度不可變的,是以不如直接采用StringBuilder(與StringBuffer 長度都是可變的,隻不過前者是非線程安全,後者是線程安全)進行字元串合并。
使用Intellij idea如何快速檢視Java類位元組碼
1、打開File-Settings
![](https://img.laitimes.com/img/__Qf2AjLwojIjJCLyojI0JCLi0zaHRGcWdUYuVzVa9GczoVdG1mWfVGc5RHLwkzX39GZhh2csATMflHLwEzX4xSZz91ZsADMx8FdsYkRGZkRG9lcvx2bjxSa2EWNhJTW1AlUxEFeVRUUfRHelRHL2EzXlpXazxyayFWbyVGdhd3LcV2Zh1Wa9M3clN2byBXLzN3btg3PnVGcq5SOlNGZ3EWYlZjNzQWZxMzY5QWYlJmZ0EDO0MTZ5QzYk9CX4AzLcdDMxIDMy8CXn9Gbi9CXzV2Zh1WavwVbvNmLvR3YxUjL3M3Lc9CX6MHc0RHaiojIsJye.jpeg)
2:打開Tools-External Tools,右側點選綠色“+”
3:填寫一些内容規則:Name是在類中,右鍵時使用時的名稱
4:代碼處右鍵,即可找到添加的功能
5、通過jdk自帶的反編譯工具指令 javap 可以檢視class檔案的位元組碼資訊
-verbose
或者
-c
都可以,詳情可以檢視javap指令如何使用