JVM Byte code

JVM

• Java SE Specifications
• The Java Virtual Machine Specification - Hotspot VM
  • 4.The class File Format
  • 6.The Java Virtual Machine Instruction Set
  • 7.Opcode Mnemonics by Opcode
• The Java Language Specification
• https://www.cnblogs.com/mengchunchen/p/7819051.html

Concept

• JVM 指令由一个字节长度的操作码和零至多个操作数构成

Java 字节码描述符

• Table 4.3-A. Interpretation of field descriptors
• 字段描述符 Field Descriptors
• 方法描述符 Method Descriptors，语法：(参数类型字段描述符)返回类型描述符

term	Type	Interpretation
B	byte	signed byte
C	char	Unicode character code,encoded with UTF-16
D	double	double-precision floating-point value
F	float	single-precision floating-point value
I	int	integer
J	long	long integer
LClassName ;	reference	an instance of class ClassName
S	short	signed short
Z	boolean	true or false
[	reference	one array dimension

由于long类型的L被引用类型占了，所以long类型用J
LClassName; 引用类型，“L” + 对象类型的全限定名 + “;”
[ 一维数组
Ljava/lang/String;  String
Ljava/util/List;    List<Order> orderList;
[Ljava/lang/Object; Object[]
[Z                  boolean[]   
[Lcom/pojo/Order;   Order[] orders;

// Method Descriptors:  ( {ParameterDescriptor} ) ReturnDescriptor
若返回类型为 void，则以 ‘V’ 结尾
注意方法名称和参数名称本身并非方法描述符的一部分，所以在表中使用了占位符
(Ljava/lang/String;)I       int method(String x)
(ILjava/lang/String;)V      void method(int x, String y)
(I)Ljava/lang/String;       String method(int x)
(Ljava/lang/String;)[C      char[] method(String x)
(ILjava/lang/String;[[Lcom/demo/FileInfo;)V     void method(int x, String y, FileInfo[][] z)

// (ILcom/pojo/Order;J)Lcom/pojo/Order;
public Order getOrder(int anInt, Order order, long aLong) {
    return order;
}
// (ILcom/pojo/Order;J)[[Lcom/pojo/Order;
public Order[][] getOrders(int anInt, Order order, long aLong) {
    return new Order[1][1];
}
// ()Ljava/util/List;
public List<Order> getOrderList() {
    return orderList;
}

Java Byte Code

jclasslib

• javap -verbose Demo // 不用带后缀.class
• 局部变量表

看到Constant Pool也就是常量池中有22项内容，其中带”Utf8″的就是符号引用。
比如#2，它的值是”com/xrq/test6/TestMain”，表示的是这个类的全限定名；又比如#5为i，#6为I，它们是一对的，表示变量时Integer（int）类型的，名字叫做i；#6为D、#7为d也是一样，表示一个Double（double）类型的变量，名字为d；#18、#19表示的都是方法的名字。

javap-v.jpg

cinit vs init

init is the (or one of the) constructor(s) for the instance, and non-static field initialization.
clinit are the static initialization blocks for the class, and static field initialization.

    public void test(int x) {
        int y = 20;
        int z = x + y;
        System.out.println(z);
    }

image.png

局部变量表：Index 0 表示 this, Index 1 表示 x

command

• 6.The Java Virtual Machine Instruction Set

整型入栈指令

• iconst_<i> Push the int constant <i> (-1, 0, 1, 2, 3, 4 or 5) onto the operand stack.
• bipush byte 取值 -128~127
• sipush byte 取值 -32768~32767
• ldc index 取值 -2147483648~2147483647