君不见长松卧壑困风霜，时来屹立扶明堂。这篇文章主要讲述Java ASM系列：（051）opcode: constant相关的知识，希望能为你提供帮助。
本文属于Java ASM系列二：OPCODE当中的一篇。
对于《java ASM系列二：OPCODE》有配套的视频讲解，可以点击这里和这里进行查看；同时，也可以点击这里查看源码资料。
1. 概览从Instruction的角度来说，与constant相关的opcode有20个，内容如下：

opcode	mnemonic symbol	opcode	mnemonic symbol	opcode	mnemonic symbol	opcode	mnemonic symbol
1	aconst_null	6	iconst_3	11	fconst_0	16	bipush
2	iconst_m1	7	iconst_4	12	fconst_1	17	sipush
3	iconst_0	8	iconst_5	13	fconst_2	18	ldc
4	iconst_1	9	lconst_0	14	dconst_0	19	ldc_w
5	iconst_2	10	lconst_1	15	dconst_1	20	ldc2_w

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

• MethodVisitor.visitInsn(): aconst_null, iconst_& lt; i& gt; , lconst_& lt; l& gt; , fconst_& lt; f& gt; , dconst_& lt; d& gt;
• MethodVisitor.visitIntInsn(): bipush, sipush
• MethodVisitor.visitLdcInsn(): ldc, ldc_w, ldc2_w

2. int 2.1. iconst_& lt; i& gt; : -1~5
从Java语言的视角，有一个HelloWorld类，代码如下：

public class HelloWorld { public void test() { int int_m1 = -1; int int_0 = 0; int int_1 = 1; int int_2 = 2; int int_3 = 3; int int_4 = 4; int int_5 = 5; } }

从Instruction的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld Compiled from "HelloWorld.java" public class sample.HelloWorld { ... public void test(); Code: 0: iconst_m1 1: istore_1 2: iconst_0 3: istore_2 4: iconst_1 5: istore_3 6: iconst_2 7: istore4 9: iconst_3 10: istore5 12: iconst_4 13: istore6 15: iconst_5 16: istore7 18: return }

从ASM的视角来看，方法体对应的内容如下：

methodVisitor.visitCode(); methodVisitor.visitInsn(ICONST_M1); methodVisitor.visitVarInsn(ISTORE, 1); methodVisitor.visitInsn(ICONST_0); methodVisitor.visitVarInsn(ISTORE, 2); methodVisitor.visitInsn(ICONST_1); methodVisitor.visitVarInsn(ISTORE, 3); methodVisitor.visitInsn(ICONST_2); methodVisitor.visitVarInsn(ISTORE, 4); methodVisitor.visitInsn(ICONST_3); methodVisitor.visitVarInsn(ISTORE, 5); methodVisitor.visitInsn(ICONST_4); methodVisitor.visitVarInsn(ISTORE, 6); methodVisitor.visitInsn(ICONST_5); methodVisitor.visitVarInsn(ISTORE, 7); methodVisitor.visitInsn(RETURN); methodVisitor.visitMaxs(1, 8); methodVisitor.visitEnd();

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

// {this} | {} 0000: iconst_m1// {this} | {int} 0001: istore_1// {this, int} | {} 0002: iconst_0// {this, int} | {int} 0003: istore_2// {this, int, int} | {} 0004: iconst_1// {this, int, int} | {int} 0005: istore_3// {this, int, int, int} | {} 0006: iconst_2// {this, int, int, int} | {int} 0007: istore4// {this, int, int, int, int} | {} 0009: iconst_3// {this, int, int, int, int} | {int} 0010: istore5// {this, int, int, int, int, int} | {} 0012: iconst_4// {this, int, int, int, int, int} | {int} 0013: istore6// {this, int, int, int, int, int, int} | {} 0015: iconst_5// {this, int, int, int, int, int, int} | {int} 0016: istore7// {this, int, int, int, int, int, int, int} | {} 0018: return// {} | {}

从JVM规范的角度来看，Operand Stack的变化如下：

... →..., < i>

Push the int constant & lt; i& gt; (-1, 0, 1, 2, 3, 4 or 5) onto the operand stack.
2.2. bipush: -128~127
从Java语言的视角，有一个HelloWorld类，代码如下：

public class HelloWorld { public void test() { int int_m128 = -128; int int_127 = 127; } }

从Instruction的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld Compiled from "HelloWorld.java" public class sample.HelloWorld { ... public void test(); Code: 0: bipush-128 2: istore_1 3: bipush127 5: istore_2 6: return }

从ASM的视角来看，方法体对应的内容如下：

methodVisitor.visitCode(); methodVisitor.visitIntInsn(BIPUSH, -128); methodVisitor.visitVarInsn(ISTORE, 1); methodVisitor.visitIntInsn(BIPUSH, 127); methodVisitor.visitVarInsn(ISTORE, 2); methodVisitor.visitInsn(RETURN); methodVisitor.visitMaxs(1, 3); methodVisitor.visitEnd();

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

// {this} | {} 0000: bipush-128// {this} | {int} 0002: istore_1// {this, int} | {} 0003: bipush127// {this, int} | {int} 0005: istore_2// {this, int, int} | {} 0006: return// {} | {}

从JVM规范的角度来看，Operand Stack的变化如下：

... →..., value

The immediate byte is sign-extended to an int value. That value is pushed onto the operand stack.
Format

bipush byte

2.3. sipush: -32768~32767
从Java语言的视角，有一个HelloWorld类，代码如下：

public class HelloWorld { public void test() { int int_m32768 = -32768; int int_32767 = 32767; } }

从Instruction的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld Compiled from "HelloWorld.java" public class sample.HelloWorld { ... public void test(); Code: 0: sipush-32768 3: istore_1 4: sipush32767 7: istore_2 8: return }

从ASM的视角来看，方法体对应的内容如下：

methodVisitor.visitCode(); methodVisitor.visitIntInsn(SIPUSH, -32768); methodVisitor.visitVarInsn(ISTORE, 1); methodVisitor.visitIntInsn(SIPUSH, 32767); methodVisitor.visitVarInsn(ISTORE, 2); methodVisitor.visitInsn(RETURN); methodVisitor.visitMaxs(1, 3); methodVisitor.visitEnd();

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

// {this} | {} 0000: sipush-32768// {this} | {int} 0003: istore_1// {this, int} | {} 0004: sipush32767// {this, int} | {int} 0007: istore_2// {this, int, int} | {} 0008: return// {} | {}

从JVM规范的角度来看，Operand Stack的变化如下：

... →..., value

Format

sipush byte1 byte2

The immediate unsigned byte1 and byte2 values are assembled into an intermediate short, where the value of the short is (byte1 & lt; & lt; 8) | byte2. The intermediate value is then sign-extended to an int value. That value is pushed onto the operand stack.
2.4. ldc: MIN~MAX
从Java语言的视角，有一个HelloWorld类，代码如下：

public class HelloWorld { public void test() { int int_min = Integer.MIN_VALUE; int int_max = Integer.MAX_VALUE; } }

从Instruction的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld Compiled from "HelloWorld.java" public class sample.HelloWorld { ... public void test(); Code: 0: ldc#3// int -2147483648 2: istore_1 3: ldc#4// int 2147483647 5: istore_2 6: return }

从ASM的视角来看，方法体对应的内容如下：

methodVisitor.visitCode(); methodVisitor.visitLdcInsn(new Integer(-2147483648)); methodVisitor.visitVarInsn(ISTORE, 1); methodVisitor.visitLdcInsn(new Integer(2147483647)); methodVisitor.visitVarInsn(ISTORE, 2); methodVisitor.visitInsn(RETURN); methodVisitor.visitMaxs(1, 3); methodVisitor.visitEnd();

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

// {this} | {} 0000: ldc#3// {this} | {int} 0002: istore_1// {this, int} | {} 0003: ldc#4// {this, int} | {int} 0005: istore_2// {this, int, int} | {} 0006: return// {} | {}

从JVM规范的角度来看，Operand Stack的变化如下：

... →..., value

Format

ldc index

The index is an unsigned byte that must be a valid index into the run-time constant pool of the current class. The run-time constant pool entry at index either must be a run-time constant of type int or float, or a reference to a string literal, or a symbolic reference to a class, method type, or method handle.
3. long 3.1. lconst_& lt; l& gt; : 0~1
从Java语言的视角，有一个HelloWorld类，代码如下：

public class HelloWorld { public void test() { long long_0 = 0; long long_1 = 1; } }

从Instruction的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld Compiled from "HelloWorld.java" public class sample.HelloWorld { ... public void test(); Code: 0: lconst_0 1: lstore_1 2: lconst_1 3: lstore_3 4: return }

从ASM的视角来看，方法体对应的内容如下：

methodVisitor.visitCode(); methodVisitor.visitInsn(LCONST_0); methodVisitor.visitVarInsn(LSTORE, 1); methodVisitor.visitInsn(LCONST_1); methodVisitor.visitVarInsn(LSTORE, 3); methodVisitor.visitInsn(RETURN); methodVisitor.visitMaxs(2, 5); methodVisitor.visitEnd();

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

// {this} | {} 0000: lconst_0// {this} | {long, top} 0001: lstore_1// {this, long, top} | {} 0002: lconst_1// {this, long, top} | {long, top} 0003: lstore_3// {this, long, top, long, top} | {} 0004: return// {} | {}

从JVM规范的角度来看，Operand Stack的变化如下：

... →..., < l>

Push the long constant & lt; l& gt; (0 or 1) onto the operand stack.
3.2. ldc2_w: MIN~MAX
从Java语言的视角，有一个HelloWorld类，代码如下：

public class HelloWorld { public void test() { long long_min = Long.MIN_VALUE; long long_max = Long.MAX_VALUE; } }

从Instruction的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld Compiled from "HelloWorld.java" public class sample.HelloWorld { ... public void test(); Code: 0: ldc2_w#3// long -9223372036854775808l 3: lstore_1 4: ldc2_w#5// long 9223372036854775807l 7: lstore_3 8: return }

【Java ASM系列（（051）opcode: constant）】从ASM的视角来看，方法体对应的内容如下：

methodVisitor.visitCode(); methodVisitor.visitLdcInsn(new Long(-9223372036854775808L)); methodVisitor.visitVarInsn(LSTORE, 1); methodVisitor.visitLdcInsn(new Long(9223372036854775807L)); methodVisitor.visitVarInsn(LSTORE, 3); methodVisitor.visitInsn(RETURN); methodVisitor.visitMaxs(2, 5); methodVisitor.visitEnd();

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

// {this} | {} 0000: ldc2_w#3// {this} | {long, top} 0003: lstore_1// {this, long, top} | {} 0004: ldc2_w#5// {this, long, top} | {long, top} 0007: lstore_3// {this, long, top, long, top} | {} 0008: return// {} | {}

从JVM规范的角度来看，Operand Stack的变化如下：

... →..., value

The numeric value of that run-time constant is pushed onto the operand stack as a long or double, respectively.
Format

ldc2_w indexbyte1 indexbyte2

The unsigned indexbyte1 and indexbyte2 are assembled into an unsigned 16-bit index into the run-time constant pool of the current class, where the value of the index is calculated as (indexbyte1 & lt; & lt; 8) | indexbyte2. The index must be a valid index into the run-time constant pool of the current class.
4. float 4.1. fconst_& lt; f& gt; : 0~2
从Java语言的视角，有一个HelloWorld类，代码如下：

public class HelloWorld { public void test() { float float_0 = 0; float float_1 = 1; float float_2 = 2; } }

从Instruction的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld Compiled from "HelloWorld.java" public class sample.HelloWorld { ... public void test(); Code: 0: fconst_0 1: fstore_1 2: fconst_1 3: fstore_2 4: fconst_2 5: fstore_3 6: return }

从ASM的视角来看，方法体对应的内容如下：

methodVisitor.visitCode(); methodVisitor.visitInsn(FCONST_0); methodVisitor.visitVarInsn(FSTORE, 1); methodVisitor.visitInsn(FCONST_1); methodVisitor.visitVarInsn(FSTORE, 2); methodVisitor.visitInsn(FCONST_2); methodVisitor.visitVarInsn(FSTORE, 3); methodVisitor.visitInsn(RETURN); methodVisitor.visitMaxs(1, 4); methodVisitor.visitEnd();

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

// {this} | {} 0000: fconst_0// {this} | {float} 0001: fstore_1// {this, float} | {} 0002: fconst_1// {this, float} | {float} 0003: fstore_2// {this, float, float} | {} 0004: fconst_2// {this, float, float} | {float} 0005: fstore_3// {this, float, float, float} | {} 0006: return// {} | {}

从JVM规范的角度来看，Operand Stack的变化如下：

... →..., < f>

Push the float constant & lt; f& gt; (0.0, 1.0, or 2.0) onto the operand stack.
4.2. ldc: MIN~MAX
从Java语言的视角，有一个HelloWorld类，代码如下：

public class HelloWorld { public void test() { float float_min = Float.MIN_VALUE; float float_max = Float.MAX_VALUE; } }

从Instruction的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld Compiled from "HelloWorld.java" public class sample.HelloWorld { ... public void test(); Code: 0: ldc#3// float 1.4E-45f 2: fstore_1 3: ldc#4// float 3.4028235E38f 5: fstore_2 6: return }

从ASM的视角来看，方法体对应的内容如下：

methodVisitor.visitCode(); methodVisitor.visitLdcInsn(new Float("1.4E-45")); methodVisitor.visitVarInsn(FSTORE, 1); methodVisitor.visitLdcInsn(new Float("3.4028235E38")); methodVisitor.visitVarInsn(FSTORE, 2); methodVisitor.visitInsn(RETURN); methodVisitor.visitMaxs(1, 3); methodVisitor.visitEnd();

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

// {this} | {} 0000: ldc#3// {this} | {float} 0002: fstore_1// {this, float} | {} 0003: ldc#4// {this, float} | {float} 0005: fstore_2// {this, float, float} | {} 0006: return// {} | {}

从JVM规范的角度来看，Operand Stack的变化如下：

... →..., value

Format

ldc index

The index is an unsigned byte that must be a valid index into the run-time constant pool of the current class. The run-time constant pool entry at index either must be a run-time constant of type int or float, or a reference to a string literal, or a symbolic reference to a class, method type, or method handle.
5. double 5.1. dconst_& lt; d& gt; : 0~1
从Java语言的视角，有一个HelloWorld类，代码如下：

public class HelloWorld { public void test() { double double_0 = 0; double double_1 = 1; } }

从Instruction的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld Compiled from "HelloWorld.java" public class sample.HelloWorld { ... public void test(); Code: 0: dconst_0 1: dstore_1 2: dconst_1 3: dstore_3 4: return }

从ASM的视角来看，方法体对应的内容如下：

methodVisitor.visitCode(); methodVisitor.visitInsn(DCONST_0); methodVisitor.visitVarInsn(DSTORE, 1); methodVisitor.visitInsn(DCONST_1); methodVisitor.visitVarInsn(DSTORE, 3); methodVisitor.visitInsn(RETURN); methodVisitor.visitMaxs(2, 5); methodVisitor.visitEnd();

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

// {this} | {} 0000: dconst_0// {this} | {double, top} 0001: dstore_1// {this, double, top} | {} 0002: dconst_1// {this, double, top} | {double, top} 0003: dstore_3// {this, double, top, double, top} | {} 0004: return// {} | {}

从JVM规范的角度来看，Operand Stack的变化如下：

... →..., < d>

Push the double constant & lt; d& gt; (0.0 or 1.0) onto the operand stack.
5.2. ldc2_w: MIN~MAX
从Java语言的视角，有一个HelloWorld类，代码如下：

public class HelloWorld { public void test() { double double_min = Double.MIN_VALUE; double double_max = Double.MAX_VALUE; } }

从Instruction的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld Compiled from "HelloWorld.java" public class sample.HelloWorld { ... public void test(); Code: 0: ldc2_w#3// double 4.9E-324d 3: dstore_1 4: ldc2_w#5// double 1.7976931348623157E308d 7: dstore_3 8: return }

从ASM的视角来看，方法体对应的内容如下：

methodVisitor.visitCode(); methodVisitor.visitLdcInsn(new Double("4.9E-324")); methodVisitor.visitVarInsn(DSTORE, 1); methodVisitor.visitLdcInsn(new Double("1.7976931348623157E308")); methodVisitor.visitVarInsn(DSTORE, 3); methodVisitor.visitInsn(RETURN); methodVisitor.visitMaxs(2, 5); methodVisitor.visitEnd();

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

// {this} | {} 0000: ldc2_w#3// {this} | {double, top} 0003: dstore_1// {this, double, top} | {} 0004: ldc2_w#5// {this, double, top} | {double, top} 0007: dstore_3// {this, double, top, double, top} | {} 0008: return// {} | {}

从JVM规范的角度来看，Operand Stack的变化如下：

... →..., value

The numeric value of that run-time constant is pushed onto the operand stack as a long or double, respectively.
6. reference type 6.1. null: aconst_null
从Java语言的视角，有一个HelloWorld类，代码如下：

public class HelloWorld { public void test() { Object obj_null = null; } }

从Instruction的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld Compiled from "HelloWorld.java" public class sample.HelloWorld { ... public void test(); Code: 0: aconst_null 1: astore_1 2: return }

从ASM的视角来看，方法体对应的内容如下：

methodVisitor.visitCode(); methodVisitor.visitInsn(ACONST_NULL); methodVisitor.visitVarInsn(ASTORE, 1); methodVisitor.visitInsn(RETURN); methodVisitor.visitMaxs(1, 2); methodVisitor.visitEnd();

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

// {this} | {} 0000: aconst_null// {this} | {null} 0001: astore_1// {this, null} | {} 0002: return// {} | {}

从JVM规范的角度来看，Operand Stack的变化如下：

... →..., null

Push the null object reference onto the operand stack.
6.2. String: ldc
从Java语言的视角，有一个HelloWorld类，代码如下：

public class HelloWorld { public void test() { String str = "GoodChild"; } }

从Instruction的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld Compiled from "HelloWorld.java" public class sample.HelloWorld { ... public void test(); Code: 0: ldc#2// String GoodChild 2: astore_1 3: return }

从ASM的视角来看，方法体对应的内容如下：

methodVisitor.visitCode(); methodVisitor.visitLdcInsn("GoodChild"); methodVisitor.visitVarInsn(ASTORE, 1); methodVisitor.visitInsn(RETURN); methodVisitor.visitMaxs(1, 2); methodVisitor.visitEnd();

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

// {this} | {} 0000: ldc#2// {this} | {String} 0002: astore_1// {this, String} | {} 0003: return// {} | {}

从JVM规范的角度来看，Operand Stack的变化如下：

... →..., value

Format

ldc index

The index is an unsigned byte that must be a valid index into the run-time constant pool of the current class. The run-time constant pool entry at index either must be a run-time constant of type int or float, or a reference to a string literal, or a symbolic reference to a class, method type, or method handle.
6.3. Class& lt; ?& gt; : ldc
从Java语言的视角，有一个HelloWorld类，代码如下：

public class HelloWorld { public void test() { Class< ?> clazz = HelloWorld.class; } }

从Instruction的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld Compiled from "HelloWorld.java" public class sample.HelloWorld { ... public void test(); Code: 0: ldc#2// class sample/HelloWorld 2: astore_1 3: return }

从ASM的视角来看，方法体对应的内容如下：

methodVisitor.visitCode(); methodVisitor.visitLdcInsn(Type.getType("Lsample/HelloWorld; ")); methodVisitor.visitVarInsn(ASTORE, 1); methodVisitor.visitInsn(RETURN); methodVisitor.visitMaxs(1, 2); methodVisitor.visitEnd();

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

// {this} | {} 0000: ldc#2// {this} | {Class} 0002: astore_1// {this, Class} | {} 0003: return// {} | {}

从JVM规范的角度来看，Operand Stack的变化如下：

... →..., value

Format

ldc index

The index is an unsigned byte that must be a valid index into the run-time constant pool of the current class. The run-time constant pool entry at index either must be a run-time constant of type int or float, or a reference to a string literal, or a symbolic reference to a class, method type, or method handle.
7. ldc and ldc_w从Java语言的视角，有一个HelloWorld类，代码如下：

public class HelloWorld { private String str001 = "str001"; // ... ... 省略255个 private String str257 = "str257"; public void test() { String str = "str258"; }public static void main(String[] args) { String format = "private String str%03d = \\"str%03d\\"; "; for (int i = 1; i < 258; i++) { String line = String.format(format, i, i); System.out.println(line); } } }

从Instruction的视角来看，方法体对应的内容如下：

$ javap -c sample.HelloWorld Compiled from "HelloWorld.java" public class sample.HelloWorld { ... public void test(); Code: 0: ldc_w#516// String str258 3: astore_1 4: return ... }

从ASM的视角来看，方法体对应的内容如下：

methodVisitor.visitCode(); methodVisitor.visitLdcInsn("str258"); methodVisitor.visitVarInsn(ASTORE, 1); methodVisitor.visitInsn(RETURN); methodVisitor.visitMaxs(1, 2); methodVisitor.visitEnd();

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

// {this} | {} 0000: ldc_w#516// {this} | {String} 0003: astore_1// {this, String} | {} 0004: return// {} | {}

从JVM规范的角度来看，Operand Stack的变化如下：

... →..., value

Format

ldc_w indexbyte1 indexbyte2

The unsigned indexbyte1 and indexbyte2 are assembled into an unsigned 16-bit index into the run-time constant pool of the current class, where the value of the index is calculated as (indexbyte1 & lt; & lt; 8) | indexbyte2. The index must be a valid index into the run-time constant pool of the current class. The run-time constant pool entry at the index either must be a run-time constant of type int or float, or a reference to a string literal, or a symbolic reference to a class, method type, or method handle.

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