上级目录

概览

从 Instruction 的角度来说，余下的 opcode 有 7 个，内容如下：

opcode mnemonic symbol opcode mnemonic symbol opcode mnemonic symbol opcode mnemonic symbol
0 nop            
opcode mnemonic symbol opcode mnemonic symbol opcode mnemonic symbol opcode mnemonic symbol
168 jsr 169 ret 201 jsr_w    
opcode mnemonic symbol opcode mnemonic symbol opcode mnemonic symbol opcode mnemonic symbol
202 breakpoint     254 impdep1 255 impdep2

从 ASM 的角度来说，这些 opcode 与 MethodVisitor.visitXxxInsn() 方法对应关系如下：

  • MethodVisitor.visitInsn(): nop
  • MethodVisitor.visitJumpInsn(): jsr, jsr_w
  • MethodVisitor.visitVarInsn(): ret

nop

Do nothing

使用 ASM 编写一段包含 nop 指向的代码如下：

import lsieun.utils.FileUtils;
import org.objectweb.asm.*;

import static org.objectweb.asm.Opcodes.*;

public class HelloWorldGenerateCore {
    public static void main(String[] args) throws Exception {
        String relative_path = "sample/HelloWorld.class";
        String filepath = FileUtils.getFilePath(relative_path);

        // (1) 生成 byte[]内容
        byte[] bytes = dump();

        // (2) 保存 byte[]到文件
        FileUtils.writeBytes(filepath, bytes);
    }

    public static byte[] dump() throws Exception {
        // (1) 创建 ClassWriter 对象
        ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);

        // (2) 调用 visitXxx()方法
        cw.visit(V1_8, ACC_PUBLIC + ACC_SUPER, "sample/HelloWorld",
                null, "java/lang/Object", null);

        {
            MethodVisitor mv1 = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
            mv1.visitCode();
            mv1.visitVarInsn(ALOAD, 0);
            mv1.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false);
            mv1.visitInsn(RETURN);
            mv1.visitMaxs(1, 1);
            mv1.visitEnd();
        }

        {
            MethodVisitor mv2 = cw.visitMethod(ACC_PUBLIC, "test", "()V", null, null);
            mv2.visitCode();
            mv2.visitInsn(NOP);
            mv2.visitInsn(NOP);
            mv2.visitInsn(NOP);
            mv2.visitInsn(NOP);
            mv2.visitInsn(NOP);
            mv2.visitInsn(RETURN);
            mv2.visitMaxs(0, 1);
            mv2.visitEnd();
        }
        cw.visitEnd();

        // (3) 调用 toByteArray()方法
        return cw.toByteArray();
    }
}

从 Instruction 的视角来看，方法体对应的内容如下：

$ javap -c -p sample.HelloWorld
public class sample.HelloWorld {
...
  public void test();
    Code:
       0: nop
       1: nop
       2: nop
       3: nop
       4: nop
       5: return
}

从 Frame 的视角来看，local variable 和 operand stack 的变化：

                               // {this} | {}
0000: nop                      // {this} | {}
0001: nop                      // {this} | {}
0002: nop                      // {this} | {}
0003: nop                      // {this} | {}
0004: nop                      // {this} | {}
0005: return                   // {} | {}

从 JVM 规范的角度来看，nop 指令对应的 Operand Stack 的变化如下：

No change

Deprecated Opcodes

If the class file version number is 51.0(Java 7) or above, then neither the jsr opcode or the jsr_w opcode may appear in the code array.

  • jsr
  • jsr_w
  • ret
Code_attribute {
    u2 attribute_name_index;
    u4 attribute_length;
    u2 max_stack;
    u2 max_locals;
    u4 code_length;
    u1 code[code_length]; // code array
    u2 exception_table_length;
    {   u2 start_pc;
        u2 end_pc;
        u2 handler_pc;
        u2 catch_type;
    } exception_table[exception_table_length];
    u2 attributes_count;
    attribute_info attributes[attributes_count];
}

Reserved Opcodes

Three opcodes are reserved for internal use by a Java Virtual Machine implementation. If the instruction set of the Java Virtual Machine is extended in the future, these reserved opcodes are guaranteed not to be used.

Two of the reserved opcodes, numbers 254 (0xfe) and 255 (0xff), have the mnemonics impdep1 and impdep2, respectively. These instructions are intended to provide “back doors” or traps to implementation-specific functionality implemented in software and hardware, respectively. The third reserved opcode, number 202 (0xca), has the mnemonic breakpoint and is intended to be used by debuggers to implement breakpoints.

Although these opcodes have been reserved, they may be used only inside a Java Virtual Machine implementation. They cannot appear in valid class files.