Java Bytecode “Portable Manual”

With manual

In Android programming, if you want to read and modify bytecode, you need to parse and translate the Class file structure. The JVM defines the Class file structure and instruction set, and you can learn the content logic of the Class file by consulting the byte code table and instruction set. Below, all the table contents and instructions involved are sorted out for inquiry.

There are two main contents: Class file structure description table and bytecode instruction table

Table references from “Understanding the Java Virtual Machine in Depth: ADVANCED JVM Specific and Best Practices”

Class file structure description table

A Class file is a compiled Java file that describes the contents of a file definition in a jVM-defined structure. The main forms are:

• Class file structure table
• Often scale
• Access tag table
• Field in the table
• Method table
• Property sheet
• List of special strings

Class file structure table

type	The name of the	describe	The number of
U4 (4 bytes)	magic	Determines whether the file is a Class file that can be accepted by the virtual machine, similar to an ID	1
U2 (2 bytes)	minot_version	Second version number	1
U2 (2 bytes)	mahor_version	The major version number	1
U2 (2 bytes)	constant_pool_count	The constant pool capacity is calculated from 1. 0 indicates that no constant pool item is referenced	1
cp_info	constant_pool	Constant pool	constant_pool_count-1
U2 (2 bytes)	access_flags	Access tokens	1
U2 (2 bytes)	this_class	Class index	1
U2 (2 bytes)	super_class	Index of the parent class	1
U2 (2 bytes)	interfaces_count	The number of interfaces implemented	1
U2 (4 bytes)	interfaces	The interface index	interfaces_count
U2 (4 bytes)	fields_count	Number of fields	1
field_info	fields	Content of the field	fields_count
U2 (2 bytes)	methods_count	Number of methods	1
method_info	methods	Method content	methods_count
U2 (2 bytes)	attributes_count	Number of attributes	1
attribute_info	attributes	Attribute content	attributes_count

Often scale

There are two main types of constant pools:

• Literals, including text strings, final constant values, and so on
• Symbol references, fully qualified names of classes and interfaces, field names and descriptors, and method names and descriptors

The Class file only holds the information of each method and field, not the memory information. The true memory entry can only be obtained through run-time transformations. When a VIRTUAL machine is running, it needs to obtain the symbolic reference from the constant pool, and then obtain the specific memory address through class creation or runtime parsing.

type	substructure	mark	describe
CONSTANT_Utf8_info	tag	u1 = 1	The character string is utF-8 encoded
–	lenght	u2	Number of bytes in a UTF-8 encoded string
–	bytes	u1	The value is a utF-8 encoded string of lenght length
CONSTANT_Integer_info	tag	u1=3	Integer literals
–	bytes	u4	Int values stored first in order of high order
CONSTANT_Float_info	tag	u1=4	Floating point literals
–	bytes	u4	Float values stored first in order of magnitude
CONSTANT_Long_info	tag	u1=5	Long integer literals
–	bytes	u8	The long value stored in front of the high order
CONSTANT_Double_info	tag	u1=6	A double – precision floating-point literal
–	bytes	u8	A double stored in front of the high order
CONSTANT_Class_info	tag	u1=7	Symbolic reference to a class or interface
–	bytes	u2	Index to a fully qualified named constant item
CONSTANT_String_info	tag	u1=8	String type literals
–	bytes	u2	Index to a string literal
CONSTANT_Fieldref_info	tag	u1=9	Symbolic reference to a field
–	index	u2	Index entry to the class or interface descriptor CONSTANT_Class_info that declares the field
–	index	u2	Index entry pointing to the class or interface descriptor CONSTANT_NameAndType_info for the declared field
CONSTANT_Methodred_info	tag	u1=10	Symbolic references to methods in a class
–	index	u2	Index entry to the class or interface descriptor CONSTANT_Class_info that declares the field
–	index	u2	Index entry pointing to the class or interface descriptor CONSTANT_NameAndType_info for the declared field
CONSTANT_InterfaceMethodref_info	tag	u1=11	Symbolic references to methods in the interface
–	index	u2	Index entry to the class or interface descriptor CONSTANT_Class_info that declares the field
–	index	u2	Index entry pointing to the class or interface descriptor CONSTANT_NameAndType_info for the declared field
CONSTANT_NameAndType_info	tag	u1=12	A partial symbolic reference to a field or method
–	index	u2	Index to the constant entry of the field or method name
–	index	u2	Index to the constant entry of the field or method name
CONSTANT_MethodHandle_info	tag	u1=15	Represents a method handle
–	reference_kind	u1	The value must be in [1,9], which determines the type of method handle. The value of the method handle type represents the bytecode behavior of the method handle
–	reference_index	u2	The value must be a valid index to the constant pool
CONSTANT_MethodType_info	tag	u1=16	Identification method type
–	descriptor_index	u2	The value must be a valid index to the constant pool, and the constant pool entry at that index must be a CONSTANT_Utf8_info structure representing the descriptor of the method
CONSTANT_InvokeDynamic_info	tag	u1=18	Represents a dynamic method call point
–	bootstrap_method_attar_index	u2	The value must be a valid index to the bootSTRAP_methods [] array of the bootstrap method table in the current Class file
–	name_and_type_index	u2	The value must be a valid index to the current constant pool, and the constant pool value at that index must be a CONSTANT_NameAndType_info structure representing the method name and method descriptor

Access tag table

The access tag table is further divided according to the following different tag types:

1. Class access flag
2. The inner class accesses the label
3. Field access flag
4. Method access flag

Class access flag

Access information that identifies some Class or interface level, including whether the Class file is a Class or interface, is defined as a public type, is defined as an abstract Class type, and, if so, is declared final.

Sign the name	Flag values	describe
ACC_PUBLIC	0x0001	Whether the type is public
ACC_FINAL	0x0010	Whether or not to be declared final can be set only by the class
ACC_SUPER	0x0020	Whether the invokespecial bytecode instruction is allowed to use the new semantics. The invokespecial instruction has changed in JDK1.0.2. In order to distinguish the semantics used in this instruction, all classes compiled after JDK1.0.2 must be true
ACC_INTERFACE	0x0200	Identify this as an interface
ACC_ABSTRACT	0x0400	The value of this flag is true for interfaces or abstract classes and false for other types
ACC_SYNTHETIC	0x1000	Identifies that this class is not generated by user code
ACC_ANNOTATION	0x2000	Mark this as an annotation
ACC_ENUM	0x4000	Identifies this as an enumeration

Inner classes access the table

Sign the name	Flag values	describe
ACC_PUBLIC	0x0001	Whether the inner class is public
ACC_PRIVATE	0x0002	Whether the inner class is private
ACC_PROTECTED	0x0004	Whether the inner class is protected
ACC_STATIC	0x0008	Whether the inner class is protected
ACC_FINAL	0x0010	Whether the inner class is protected
ACC_INTERFACE	0x0020	Whether the inner class is an interface
ACC_ABSTRACT	0x0400	Whether the inner class is abstract
ACC_SYNTHETIC	0x1000	Whether an inner class is not generated by user code
ACC_ANNOTATION	0x2000	Whether the inner class is an annotation
ACC_ENUM	0x4000	Whether the inner class is an enumeration

Field access flag

Sign the name	Flag values	describe
ACC_PUBLIC	0x0001	Whether the field is public
ACC_PRIVATE	0x0002	Whether the field is private
ACC_PROTECTED	0x0004	Whether the field is protected
ACC_STATIC	0x0008	Whether the field is static
ACC_FINAL	0x0010	Whether the field is final
ACC_VOLATILE	0x0040	Whether the field is volatile
ACC_TRANSIENT	0x0080	Whether the field is TRANSIENT
ACC_SYNTHETIC	0x1000	Whether the field is automatically generated by the compiler
ACC_ENUM	0x4000	Whether the field is enum

Method access flag

Sign the name	Flag values	describe
ACC_PUBLIC	0x0001	Whether the method is public
ACC_PRIVATE	0x0002	Whether the method is private
ACC_PROTECTED	0x0004	Whether the method is protected
ACC_STATIC	0x0008	Whether the method is static
ACC_FINAL	0x0010	Whether the method is final
ACC_SYNCHRONIZED	0x0020	Whether the method is synchronized
ACC_BRIDGE	0x0040	Method is the bridge method generated by the compiler
ACC_VARARGS	0x0080	Whether a method accepts an indefinite parameter
ACC_NATIVE	0x0100	Whether the method is native
ACC_ABSTRACT	0x0400	Whether the method is abstract
ACC_STRICTFP	0x0800	Whether the method is strictFP
ACC_SYNTHETIC	0x1000	Whether a method is generated automatically by the compiler

Field in the table

Used to describe variables declared in interfaces and classes, including class-level and instance-level variables.

type	The name of the	The number of
u2	access_flags	1
u2	name_index	1
u2	descriptor_index	1
u2	attributes_count	1
u2	attributes	attributes_count
Among themaccess_flagsSee field access flags for Access flags table.

Method table

The method table contains access flags, name indexes, descriptor indexes, and property tables

type	The name of the	The number of
u2	access_flags	1
u2	name_index	1
u2	descriptor_index	1
u2	attributes_count	1
attribute_info	attributes	attributes_count
Methodologicalaccess_flagsSee method access flags above

Property sheet

Property sheets are tables that explain properties carried in Class files, field tables, and method tables. Properties are used to describe information specific to certain scenarios.

The attribute name	Use location	meaning
Code	Method table	Bytecode instructions compiled into Java code
ConstantValue	Field in the table	Constant value defined by the final keyword
Deprecated	Class, method table, field table	Constant value defined by the final keyword
Exceptions	Method table	An exception thrown by the final method
EnclosingMethod	The class file	This property is only available if a class is local or anonymous, and is used to identify the enclosing method of the class
InnerClasses	The class file	Inner class list
LineNumberTable	Code attributes	The mapping of Java source line numbers to bytecode instructions
LocalVariableTable	Code attributes	Method local variable description
StackMapTable	Code attributes	New property in JDK1.6 for the new Type Checker to check and process whether the local variable of the target method matches the Type required by the operand stack lock
Signature	Class, method table, field table	In the Java language, the generic signature of any class, interface, initializer, or member that contains Type Variables or Parameterized Types. The Signature attribute records generic Signature information for it. Because Java generics are implemented by erasure, this property is needed to record relevant information in the generics in order for the type information to be erased to cause confusion in the signature
SourceFile	The class file	Record the source file name
SourceDebugExtension	The class file	A new property in JDK1.6, the SourceDebugExtension property is used to store additional debugging information. For example, when debugging JSP files, the Java stack cannot be used to locate the line number of a JSP file. The JSR-45 specification provides a standard mechanism for debugging programs that are not written in Java but need to be compiled into bytecode and run in the Java virtual machine. The SourceDebugExtension property can be used to store debugging information added to the standard
Synthetic	Class, method table, field table	Identifies whether a method or field is automatically generated by the compiler
LocalVariableTypeTable	class	New properties in JDK1.5, which use signature signatures instead of descriptors, were added to describe generic parameterized types after the introduction of generic syntax
RuntimevisibleAnnotations	Class, method table, field table	New properties in JDK1.5 to provide support for dynamic annotations. RuntimevisibleAnnotations property is used to specify which annotations are runtime (actually the runtime is reflection calls)
RuntimeInvisibleAnnotations	Class, method table, field table	JDK1.5 new attributes, in contrary to RuntimevisibleAnnotations attribute function, used to indicate which notes are not visible at runtime
RuntimeVisibleParameterAnnotations	Method table	New attribute, role in JDK1.5 like RuntimevisibleAnnotations attribute, but function object for method parameters
RuntimeInvisibleParameterAnnotations	Method table	New attribute, role in JDK1.5 like RuntimeInvisibleAnnotations attribute, but function object for method parameters
AnnotationDetault	Method table	Attribute added in JDK1.5 to record default values for annotation class elements
BootstrapMethods	The class file	Property added in JDK1.5 to hold bootstrap method qualifiers for invokeDynamic directive references

Each of the above attributes needs to be identified by referencing a constant of type CONSTANT_Utf8_info from the constant pool. It also contains attribute_length (u4), which identifies the number of digits occupied by the attribute value, followed by the attribute content.

Code attribute structure

type	The name of the	The number of
u2	attribute_name_index	1
u4	attribute_length	1
u2	max_stack	1
u2	max_locals	1
u4	code_length	1
u1	code	code_lenght
u2	exception_table_lenght	1
exception_info	exception_table	exception_table_length
u2	attributes_count	1
attribute_info	attributes	attributes_count

Exception attribute structure

type	The name of the	The number of
u2	start_pc	1
u2	end_pc	1
u2	handler_pc	1
u2	catch_type	1

Exceptions attribute structure

This table is different from the exception table, which is the checked exceptions that may be thrown by methods in the listing, that is, the exceptions listed by the keyword when method description

type	The name of the	The number of
u2	attribute_name_index	1
u4	attribute_length	1
u2	number_of_exceptions	1
u2	exception_index_table	number_of_exceptions

LineNumberTable Property structure

Used to describe the mapping between Java source line numbers and bytecode line numbers, generated by default in a Class file.

type	The name of the	The number of
u2	attribute_name_index	1
u4	attribute_length	1
u2	line_number_table_length	1
line_number_info	line_number_table	line_number_table_length
Line_number_info contains two u2-type data items, start_PC and line_number.

LocalVariableTable property structure

Used to describe the relationship between variables in the local variable table in the stack frame and variables defined in the Java source code, generated by default to a Class file.

type	The name of the	The number of
u2	attribute_name_index	1
u4	attribute_length	1
u2	local_variable_table_lenght	1
local_variable_info	local_variable_table	local_variable_table_lenght

Where local_variable_info represents the association between stack frames and local variables in source code, as shown in the following table:

type	The name of the	meaning	The number of
u2	start_pc	The bytecode offset at the start of the life cycle of a local variable	1
u2	length	The life cycle of a local variable begins with scope coverage length	1
u2	name_index	Points to the constant pool index CONSTANT_Utf8_info	1
u2	descriptor_index	Points to the constant pool index CONSTANT_Utf8_info	1
u2	index	The Slot position of a local variable in the stack frame local variable table	1

SourceFile attribute structure

Record the name of the source file that generated the Class file.

type	The name of the	The number of
u2	attribute_name_index	1
u4	attribute_length	1
u2	sourcefile_index	1

Sourcefile_index is the index pointing to the constant pool CONSTANT_Utf8_info.

ConstantValue attribute structure

Used to tell the virtual machine to automatically assign values to static variables. Only variables decorated with the static keyword can use this property.

type	The name of the	The number of
u2	attribute_name_index	1
u4	attribute_length	1
u2	constant_index	1

InnerClasses property structure

Used to record associations between inner classes and host classes. If an inner class is defined in a class, the compiler generates the InnerClasses attribute for it.

type	The name of the	The number of
u2	attribute_name_index	1
u4	attribute_length	1
u2	number_of_classes	1
inner_classes_info	inner_classes	number_of_classes

Each inner_classes_info represents an inner class, structured as follows:

type	The name of the	meaning	The number of
u2	inner_class_info_index	Points to the constant pool CONSTANT_Class_info index	1
u2	outer_class_info_index	Points to the constant pool CONSTANT_Class_info index	1
u2	inner_name_index	Points to the constant pool CONSTANT_Utf8_info index, which represents the name of this inner class, or 0 if anonymous	1
u2	inner_class_access_flags	For access flags for inner classes, see the access flags section above	1

Deprecated/Synthetic attribute structure

The former is used to indicate whether a class, field, or method is no longer recommended.

The latter is used to indicate that a field or method is not directly generated by Java source code. All methods generated by non-user code require the Synthetic property or ACC_SYNTHETIC flag to be set, except for

and

. Their structure is as follows:

type	The name of the	The number of
u2	attribute_name_index	1
u4	attribute_length	1

StackMapTable attribute structure

Added to the Class file specification after JDK1.6 in the Code property table, this property is used by the Type Checker during the bytecode verification phase of the virtual machine Class load.

type	The name of the	The number of
u2	attribute_name_index	1
u4	attribute_length	1
u2	number_of_entries	1
stack_map_frame	stack_map_frame_entries	number_of_entries

Signature attribute structure

Added to the Class file specification after the JDK1.5 release, it is an optional fixed-length property that can appear in the property tables of classes, property tables, and method table structures. This property records the signature information of generics, which in the Java language are pseudo-generics implemented by erasure. In bytecode properties, the generics information is erased after compilation. This property was added to compensate for the inability of runtimes like C# to support retrieving true generic types, and Java reflection can now retrieve generic types.

type	The name of the	The number of
u2	attribute_name_index	1
u4	attribute_length	1
u2	signature_index	1

Signature_index must be a valid index to the constant pool and be CONSTANT_Utf8_info, indicating the class signature, method type signature, or field type signature. This structure represents the class Signature if the current Signature attribute is an attribute of the class file, the method type Signature if the current Signature attribute is an attribute of the method table, and the field type Signature if the current Signature attribute is an attribute of the field table.

BootstrapMethods attribute structure

Added to the Class file specification after JDK1.7 was released, is a complex variant-length property that resides in the property sheet of the Class file.

type	The name of the	The number of
u2	attribute_name_index	1
u4	attribute_length	1
u2	num_bootstrap_methods	1
bootstrap_method	bootstrap_methods	num_bootstrap_methods

The bootSTRap_method structure is as follows

type	The name of the	The number of
u2	bootstrap_method_ref	1
u2	num_bootstrap_arguments	1
u2	bootstrap_arguments	num_bootstrap_arguments

List of special strings

A fully qualified name is a class name separated by a “.”. For example, com/yummylau/TestClass replaces “.” with “/” to become com.yummylau.TestClass. Segmentation. Simple names are methods or field names without type and parameter modifications, such as method inc() and field M, identified as inc and m, respectively. Special string tables contain descriptions of some basic types and method descriptions. As follows:

• The descriptor

Identification character	meaning
B	Basic type byte
C	Base type char
D	Base type double
F	Base type float
I	Basic int
J	Base type long
S	Basic type short
Z	Basic type Boolean
V	Base type void
L	Object types, such as Ljava/lang/Object

For arrays, each dimension is described with a leading “[” character, such as defining a “java.lang.string [][]” array, which is recorded as “[[java.lang.string; “and an integer array” int[] “, which is recorded as [I for methods

Methods the scene	The descriptor
Void inc ()	(V)
Java. Lang. String toString ()	() Ljava/lang/String;
Int indexOf (char[]source, int sourceOffest, int sourceCount, char[] target, int targetOffset, int targetCOunt, int formIndex)	([CII [CIII) I

Table of bytecode instructions

According to the type of instruction/target data type/common instruction, it can be further divided into the following contents:

• Summary table of bytecode instructions
• Conversion of data types in instructions
• The data type supported by the instruction set
• Load/store instructions
• Operation instruction
• Type conversion instruction
• Object creation and access directives
• Operand stack management instructions
• Control transfer instruction
• Method calls and return directives
• Exception handling instruction
• Synchronization instructions

Summary table of bytecode instructions

Tip: Refer to the official JVM directive documentation for a more detailed description

The bytecode	mnemonics	Instruction meaning
0x00	nop	Do nothing
0x01	aconst_null	Push NULL to the top of the stack
0x02	iconst_m1	Push int -1 to the top of the stack
0x03	iconst_0	Push int 0 to the top of the stack
0x04	iconst_1	Push int 1 to the top of the stack
0x05	iconst_2	Push int 2 to the top of the stack
0x06	iconst_3	Push int 3 to the top of the stack
0x07	iconst_4	Push int 4 to the top of the stack
0x08	iconst_5	Push int 5 to the top of the stack
0x09	lconst_0	Push long 0 to the top of the stack
0x0a	lconst_1	Push long 1 to the top of the stack
0x0b	fconst_0	Push type float 0 to the top of the stack
0x0c	fconst_1	Push type float 1 to the top of the stack
0x0d	fconst_2	Push type float 2 to the top of the stack
0x0e	dconst_0	Push double 0 to the top of the stack
0x0f	dconst_1	Push double 1 to the top of the stack
0x10	bipush	Push a one-byte constant (-128-127) to the top of the stack
0x11	sipush	Push a short integer constant constant (-32768-32767) to the top of the stack
0x12	ldc	Pushes int, float, String constants from the constant pool to the top of the stack
0x13	ldc_w	Push int, float, String constant values from the constant pool to the top of the stack (wide index)
0x14	ldc2_w	Push long or float constant values from the constant pool to the top of the stack (wide index)
0x15	iload	Pushes the specified local variable of type int to the top of the stack
0x16	lload	Pushes the specified long local variable to the top of the stack
0x17	fload	Pushes the specified local variable of type float to the top of the stack
0x18	dload	Pushes the specified dload local variable to the top of the stack
0x19	aload	Pushes the specified reference type local variable to the top of the stack
0x1a	iload_0	Push the first local variable of int to the top of the stack
0x1b	iload_1	Push the second local variable of int to the top of the stack
0x1c	iload_2	Push the third local variable of type int to the top of the stack
0x1d	iload_3	Push the fourth local variable of int to the top of the stack
0x1e	lload_0	Push the first long local variable to the top of the stack
0x1f	lload_1	Push the second long local variable to the top of the stack
0x20	lload_2	Push the third long local variable to the top of the stack
0x21	lload_3	Push the fourth long local variable to the top of the stack
0x22	fload_0	Push the first local variable of type float to the top of the stack
0x23	fload_1	Push the second local variable of type float to the top of the stack
0x24	fload_2	Push the third local variable of type float to the top of the stack
0x25	fload_3	Push the fourth local variable of type float to the top of the stack
0x26	dload_0	Push the first double local variable to the top of the stack
0x27	dload_1	Push the second double local variable to the top of the stack
0x28	dload_2	Push the third double local variable to the top of the stack
0x29	dload_3	Push the fourth local variable of type double to the top of the stack
0x2a	aload_0	Push the first reference type local variable to the top of the stack
0x2b	aload_1	Push the second reference type local variable to the top of the stack
0x2c	aload_2	Push the third reference type local variable to the top of the stack
0x2d	aload_3	Push the fourth reference type local variable to the top of the stack
0x2e	iaload	Pushes the value of the specified index of an int to the top of the stack
0x2f	laload	Pushes the value of the specified index of the long array to the top of the stack
0x30	faload	Pushes the value of the specified index of a float array to the top of the stack
0x31	daload	Pushes the value of the specified index of an array double to the top of the stack
0x32	aaload	Pushes the value of the specified index of a reference array to the top of the stack
0x33	baload	Pushes the value of a Boolean or Byte array specified index to the top of the stack
0x34	caload	Pushes the value of a char array to the top of the stack
0x35	saload	Pushes the value of the specified index of the short array to the top of the stack
0x36	istore	Stores the top int value to the specified local variable
0x37	lstore	Stores the top of the stack long value to the specified local variable
0x38	fstore	Stores the top stack float value to the specified local variable
0x39	dstore	Stores the top of the stack double to the specified local variable
0x3a	astore	Stores the top of the stack reference value to the specified local variable
0x3b	istore_0	Store the top int value to the first local variable
0x3c	istore_1	Store the top int value to the second local variable
0x3d	istore_2	Store the top int value into the third local variable
0x3e	istore_3	Store the top int value into the fourth local variable
0x3f	lstore_0	Store the top of the stack value into the first local variable
0x40	lstore_1	Store the top of the stack long value into the second local variable
0x41	lstore_2	Store the top of the stack long value into the third local variable
0x42	lstore_3	Store the top of the stack long value into the fourth local variable
0x43	fstore_0	Store the top stack float value to the first local variable
0x44	fstore_1	Store the top stack float value into the second local variable
0x45	fstore_2	Store the top stack float value into the third local variable
0x46	fstore_3	Store the top stack float value into the fourth local variable
0x47	dstore_0	Store the top of the stack double to the first local variable
0x48	dstore_1	Store the top of the stack double to the second local variable
0x49	dstore_2	Store the top of the stack double into the third local variable
0x4a	dstore_3	Store the top of the stack double into the fourth local variable
0x4b	astore_0	Store the top of the stack reference value into the first local variable
0x4c	astore_1	Store the top of the stack reference value into the second local variable
0x4d	astore_2	Store the top of the stack reference value into a third local variable
0x4e	astore_3	Store the top of the stack reference value into the fourth local variable
0x4f	iastore	Stores the top int into the specified index position of the specified array
0x50	lastore	Stores the top long value to the specified index position in the specified array
0x51	fastore	Stores the top float value to the specified index position of the specified array
0x52	dastore	Stores the top double to the specified index position in the specified array
0x53	aastore	Stores the top of the stack reference value to the specified index position of the specified array
0x54	bastore	Stores the top stack Boolean or Byte value to the specified index position of the specified array
0x55	castore	Stores the top char value to the specified index position in the specified array
0x56	sastore	Stores the top short value to the specified index position in the specified array
0x57	pop	Pop top value (value cannot be long or double)
0x58	pop_2	Pops one (for long or double) or two values (for other types that are not long or double) at the top of the stack
0x59	dup	Copies the top value of the stack and pushes it to the top
0x5a	dup_x1	Copies the top value of the stack and pushes the two replicated values to the top
0x5b	dup_x2	Copy the top value and push three (or two) copy values to the top of the stack
0x5c	dup_2	Copy one (for long or double) or two (for other types other than long or double) values to the top of the stack and push the copy value to the top of the stack.
0x5d	dup_2_x1	Double version of the dup_x1 instruction
0x5e	dup_2_x2	Double version of the dup_x2 instruction
0x5f	swap	Swap the two values at the top of the stack (values cannot be long or double)
0x60	iadd	Add two int values at the top of the stack and push the result to the top
0x61	ladd	Add the top two longs and push the result to the top of the stack
0x62	fadd	Add two float values at the top of the stack and push the result to the top
0x63	dadd	Add the top two double values and push the result to the top of the stack
0x64	isub	Subtract the top two int values and push the result to the top of the stack
0x65	lsub	Subtract the top two longs and push the result to the top of the stack
0x66	fsub	Subtract the top two float values and push the result to the top of the stack
0x67	dsub	Subtract the top double and push the result to the top of the stack
0x68	imul	Multiply the top two int values and push the result to the top of the stack
0x69	lmul	Multiply the top two longs and push the result to the top of the stack
0x6a	fmul	Multiply the top two float values and push the result to the top of the stack
0x6b	dmul	Multiply the top two double values and push the result to the top of the stack
0x6c	idiv	Divide the top two int values and push the result to the top of the stack
0x6d	ldiv	Divide the top two longs and push the result to the top of the stack
0x6e	fdiv	Divide two float values at the top of the stack and push the result to the top
0x6f	ddiv	Divide the top double and push the result to the top of the stack
0x70	irem	The two int values at the top of the stack are modelled and the results are pushed to the top of the stack
0x71	lrem	The two long values at the top of the stack are used for modular operation and the result is pushed to the top of the stack
0x72	frem	Modulo the top two float values and push the result to the top of the stack
0x73	drem	Modulo operation is performed on the top two double values and the result is pushed to the top of the stack
0x74	ineg	Negates the top two ints and pushes the result to the top of the stack
0x75	lneg	Negate the top two longs and push the result to the top of the stack
0x76	fneg	Negates the top two float values and pushes the result to the top of the stack
0x77	dneg	Negates the top double and pushes the result to the top
0x78	ishl	Shifts the top two ints to the left by the specified number of digits and pushes the result to the top of the stack
0x79	lshl	Shifts the top two longs to the left by the specified number of digits and pushes the result to the top of the stack
0x7a	ishr	Shifts (signed) the specified number of digits right of the top two int values and pushes the result to the top of the stack
0x7b	lshr	Shifts the top two longs right (signed) by the specified number of digits and pushes the result to the top of the stack
0x7c	iushr	Shifts the top two int values to the right (unsigned) by the specified number of digits and pushes the result to the top of the stack
0x7d	lushr	Shifts the top two longs right (unsigned) by the specified number of digits and pushes the result to the top of the stack
0x7e	iand	The top two int values are “bitwise and” and pushed to the top of the stack
0x7f	land	The two longs at the top of the stack are bitwise and pushed to the top of the stack
0x80	ior	Place the top two ints as bitwise or and push the result to the top of the stack
0x81	lor	The top two longs are “bitwise or” and pushed to the top of the stack
0x82	ixor	The top two int values are bitwise xor and pushed to the top of the stack
0x83	lxor	The top two longs are bitwise xor and the result is pushed to the top of the stack
0x84	iinc	Increments a top int variable to the specified value (I ++, I –, I +=2, etc.)
0x85	i2l	Forces the top int value to a long value and pushes the result to the top of the stack
0x86	i2f	Forces the top int value to a float value and pushes the result to the top of the stack
0x87	i2d	Forces the top int to a double and pushes the result to the top of the stack
0x88	l2i	Forces the top long value to an int value and pushes the result to the top of the stack
0x89	l2f	Forces the top of the stack long value to float and pushes the result to the top of the stack
0x8a	l2d	Forces the top long into a double and pushes the result to the top of the stack
0x8b	f2i	Force a top float value to an int value and push the result to the top of the stack
0x8c	f2l	Force a top float value to a long value and push the result to the top of the stack
0x8d	f2d	Forces a top float value to a double value and pushes the result to the top of the stack
0x8e	d2i	Forces the top double to an int and pushes the result to the top of the stack
0x8f	d2l	Forces the top double to a long and pushes the result to the top of the stack
0x90	d2f	Forces the top double to float and pushes the result to the top of the stack
0x91	i2b	Forces the top int value to a byte value and pushes the result to the top of the stack
0x92	i2c	Forces the top int value to a char value and pushes the result to the top of the stack
0x93	i2s	Forces the top int to a short value and pushes the result to the top of the stack
0x94	lcmp	Compare the two longs at the top of the stack and push the result (1, 0, or -1) to the top of the stack
0x95	fcmpl	Compare the size of two float values at the top of the stack and push the result (1, 0, or -1) to the top of the stack; When one of the values is “NaN”, -1 is pushed to the top of the stack
0x96	fcmpg	Compare the size of two float values at the top of the stack and push the result (1, 0, or -1) to the top of the stack; When one of the values is “NaN”, 1 is pushed to the top of the stack
0x97	dcmpl	Compare two double values at the top of the stack and push the result (1, 0, or -1) to the top of the stack; When one of the values is “NaN”, -1 is pushed to the top of the stack
0x98	dcmpg	Compare two double values at the top of the stack and push the result (1, 0, or -1) to the top of the stack; When one of the values is “NaN”, 1 is pushed to the top of the stack
0x99	ifeg	Jump if the top value of int equals 0
0x9a	ifne	Jump if the top value of int does not equal 0
0x9b	iflt	Jump if the top value of int is less than 0
0x9c	ifge	Jump if the top value of int is greater than or equal to 0
0x9d	ifgt	Jump if the top value of int is greater than 0
0x9e	ifle	Jump if the top value of int is less than or equal to 0
0x9f	if_icmpeq	Compares the size of two ints at the top of the stack, jumps when the result equals 0
0xa0	if_icmpne	Compares the size of two ints at the top of the stack, jumps if the result is not equal to 0
0xa1	if_icmplt	Compares the size of two ints at the top of the stack, jumps if the result is less than 0
0xa2	if_icmpge	Compares the size of two ints at the top of the stack, jumps if the result is greater than or equal to 0
0xa3	if_icmpgt	Compares the size of two ints at the top of the stack, jumps if the result is greater than 0
0xa4	if_icmple	Compares the size of two ints at the top of the stack, jumps if the result is less than or equal to 0
0xa5	if_icmpeq	Compares two referential values at the top of the stack, jumps if the results are equal
0xa6	if_icmpnc	Compares two referential values at the top of the stack, jumps if the results are not equal
0xa7	goto	Unconditional jump
0xa8	jsr	Jumps to the specified 16-bit offset and pushes the address of the next JSR instruction to the top of the stack
0xa9	ret	Returns the instruction location to the index specified by the local variable (usually used in conjunction with JSR or jSR_w)
0xaa	tableswitch	For switch conditional jump, case value continuous (variable length instruction)
0xab	lookupswitch	For switch conditional jump, case value not continuous (variable length instruction)
0xac	ireturn	Returns an int from the current method
0xad	lreturn	Returns long from the current method
0xae	freturn	Returns float from the current method
0xaf	dreturn	Returns double from the current method
0xb0	areturn	Returns an object reference from the current method
0xb1	return	Returns void from the current method
0xb2	getstatic	Gets the static field of the specified class and pushes its value to the top of the stack
0xb3	putstatic	Assigns a value to the static field of the specified class
0xb4	getfield	Gets the instance domain of the specified class and pushes its value to the top of the stack
0xb5	putfield	Assigns a value to the instance field of the specified class
0xb6	invokevirtual	Calling instance methods
0xb7	invokespecial	Call superclass constructor, instance initializer, private method
0xb8	invokestatic	Calling static methods
0xb9	invokeinterface	Calling interface methods
0xba	invokedynamic	Calling dynamic methods
0xbb	new	Creates an object and pushes its reference to the top of the stack
0xbc	newarray	Creates an array of the specified primitive type (such as int, float, etc.) and pushes its reference value to the top of the stack
0xbd	anewarray	Creates an array of references (such as classes, interfaces, arrays) and pushes its reference values to the top of the stack
0xbe	arraylength	Get the length of the array and push it to the top of the stack
0xbf	athrow	Throws the exception at the top of the stack
0xc0	checkcast	Check for type conversions, failing which throws a ClassCastException
0xc1	instanceof	Checks if the object is an instance of the specified class, if so, 1 is pushed to the top of the stack, otherwise 0 is pushed to the top
0xc2	monitorenter	Gets the lock of an object that is used to synchronize a method or block
0xc3	monitorexit	Release the lock of an object for synchronizing methods or blocks
0xc4	wide	Expand the width of a local variable
0xc5	multianewarray	Creates a multidimensional array of the specified type and the specified dimension (to execute this instruction, the stack must contain the length values for each dimension) and pushes its reference values to the top of the stack
0xc6	ifnull	If the value is null, it jumps
0xc7	ifnonnull	The value is not null
0xc8	goto_w	Unconditional jump (wide index)
0xc9	jsr_w	Jumps to the specified 32-bit offset position and pushes the next instruction address of jSR_W to the top of the stack

Conversion of data types in instructions

The data type	byte	short	int	long	float	double	char	reference
Simplified conversion	b	s	i	l	f	d	c	a

The data type supported by the instruction set

The T+ instruction in the following table constitutes Opcode, and T is the simplified transformation of each data type in the above table.

opcode	byte	short	int	long	float	double	char	reference
Tipush	bipush	sipush
Tconst			iconst	lconst	fconst	dconst		aconst
Tload			iload	lload	fload	dload		aload
Tstore			istore	lstore	fstore	dstore		astore
Tinc				iinc
Taload	baload	saload	iaload	laload	faload	daload	caload	aaload
Tastore	bastore	sastore	iastore	lastore	fastore	dastore	castore	aastore
Tadd			iadd	ladd	fadd	dadd
Tsub			isub	lsub	fsub	dsub
Tmul			imul	lmul	fmul	dmul
Tdiv			idiv	ldiv	fdiv	ddiv
Trem			irem	lrem	frem	drem
Tneg			ineg	lneg	fneg	dneg
Tshl			ishl	lshl
Tshr			ishr	lshr
Tushr			iushr	lushr
Tand			iand	land
Tor			ior	lor
Txor			ixor	lxor
i2T	i2b	i2s		i2l	i2f	i2d
l2T			l2i		l2f	l2d
f2T			f2i	f2l		f2d
d2T			d2i	d2l	d2f
Tcmp				lcmp
Tcml					fcml	dcml
Tcmpg					fcmpg	dcmpg
if_TcmpOP			if_icmpOP					if_acopOP
Treturn			ireturn	lreturn	freturn	dreturn		areturn

Most instructions do not support byte, char, short or even Boolean, and the compiler extends these types of data to int at compiler or runtime.

Load/store instructions

Load/store instructions are used to transfer data back and forth between the local variable table and operand stack in a stack frame.

• Load a local variable onto the stack:Tload ， Tload_nThe latter means a set of instructions.
• To store a value from the operand stack to a local variable table:Tstore ， Tstore_nThe latter means a set of instructions.
• Load a constant onto the operand stack:Tipush ， ldc ，T_const 等
• Extended local variable table access index instruction:wide

Operation instruction

Perform an operation on the operand stack and store the result back on top of the operand stack.

• Add instructionTadd
• Subtraction instructionsTsub
• Multiplication instructionsTmul
• Division instructionsTdiv
• For more instructionTrem
• Take the instructionTneg
• Displacement instructionsTshl.Tshr.Tushr
• By bit or instructionTor
• By bit and instructionTand
• Bitwise xOR instructionTxor
• Local variable increment instructionTinc
• More instructionsTcmpg ,Tcmpl

Type conversion instruction

Conversion instructions are used to convert two different numeric types to each other. Such conversion operations are typically used to implement explicit conversion operations in user code, or to deal with the problem of data type related instructions in the bytecode instruction set that do not have a one-to-one mapping to the data type.

• Int type to otheri2T
• Type long to otherl2T
• Float type to otherf2T
• Type double to otherd2T

Object creation and access directives

Although class instances and arrays are both objects, the Java virtual machine uses different bytecode instructions to create and manipulate class instances and arrays.

• Creating a class instancenew
• Create an arraynewarray.anewarray.multianewarray
• Access class variables and instance variablesgetfield.putfield.getstatic.putstatic
• Load an array element onto the operand stackTaload
• Stores the values of an operand stack into an array elementTastore
• The instruction to take the length of an arrayarraylength
• Check the class instance typeinstanceof.checkcast

Operand stack management instructions

• Push one or two elements off the top of the operand stackpop.pop2
• Copies one or two values from the top of the stack and pushes them back to the topdup.dup2.dup_x1.dup2_x1.dup_x2.dup2_x2
• Swap the top two values on the stackswap

Control transfer instruction

Allow the virtual machine to execute programs conditionally or unconditionally from a specific location rather than at the next instruction that controls the transfer instruction.

• Conditional branchifeq.ifit.ifle.ifne.ifgt.ifge.ifull.ifnonnull.if_icmpeq.if_icmpne.if_icmplt.if_icmpgt.if_icmple.if_icmpge.if_acmpeq.if_acmpne
• Compound conditional branchtableswitch.lookupswitch
• Unconditional branchgoto.goto_w.jsr.jsr_w.ret

Method calls and return directives

• Invokes the instance method of the objectinvokevirtualAccording to the actual type of the object
• Calling interface methodsinvokeinterface, will search at runtime for an object that implements a method of this interface and find the appropriate method to call
• Call some instance methods that require special handlinginvokespecialIncluding instance initializer methods, private methods, and superclass methods
• Calling class methodsinvokestaticUsed to call static methods
• The runtime dynamically parses the method referenced by the call point qualifier at call point and executes itinvokedynamicUnlike the previous four instructions, which are hardwired into the JVM, the dispatch logic for this instruction is determined by the boot method set by the user.

Exception handling instruction

The athrow directive is used to perform the operation of explicitly throwing an exception (throw). In addition to throwing an exception, the JVM also specifies that it will be thrown automatically at runtime when other JVM instructions detect an exception. For example, the JVM throws the ArithmeticException in idiV or Ldiv when the divisor is zero.

Synchronization instructions

The JVM uses Monitor to support the following synchronization scenarios:

• Method level synchronization, which does not require bytecode control, is implemented for method calls and return operation macros. ACC_SYNCHRONIZED from the method table shows whether a method is synchronized. If set, the thread of execution needs to hold the pipe before it can execute and release the pipe when it is finished.
• The synchronization of a sequence of instructions within a methodmonitorenter 和 monitorexitTo support synchronized functionality.

If you use Java bytecode file structures and instructions, save them

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