1. The concept
To start with a review of the Java memory model, the Java runtime data area is roughly divided into five blocks
- Methods area
- The virtual machine stack
- Local method stack
- The heap
- Program counter
The runtime constant pool is part of the method area.
The Runtime Constant Pool is part of the method area. In addition to the Class version, field, method, and interface descriptions, the Class file contains the ConstantPool Table, which is used to store the various literals and symbolic references generated by the compiler. This part of the Table is stored in the runtime ConstantPool of the method area after the Class is loaded.
The runtime constant pool contains data loaded from the constant pool table in the Class file. To find out what is in the runtime constant pool table, we need to ask what is in the corresponding constant pool table
2. Constant pool table
2.1 Data type of the Class file
The file format of the Class file uses a pseudo-structure similar to the C-language structure to store data. There are only two types of data in the pseudo-structure, “unsigned number” and “table.”
- Unsigned numbers are basic data types. U1 / U2 / U4 / U8 represent unsigned numbers of 1 byte, 2 byte, 4 byte, and 8 byte respectively. Unsigned numbers can be used to describe numbers, index references, quantity values, or string values encoded in UTF-8.
- A table is a data type that consists of multiple unsigned numbers or other tables as data items. To facilitate differentiation, all table names are customarily terminated with “_info”. Tables are used to describe hierarchical composite structures of data, and the entire Class file can be viewed as essentially a table.
2.2 constant pool
A constant pool can be likened to a repository of resources in a Class file. It is the data that is most associated with other items in the Class file structure.
The constant pool contains two main pieces of data: literals and symbolic references. Literals are close to the Java language level concepts of constants, such as text strings, constant values declared final, and so on.
2.2.1 Symbolic Reference
Since Java code does not “wire” during Javac compilation as C/C++ does, it is dynamically wired when the Clsss file is loaded by the VIRTUAL machine, the Class file does not hold the in-memory layout of methods, fields, and so on after we compile Java code into a Class file. To solve this problem, Class files hold symbolic references to methods, fields, and so on in the constant pool. Symbolic references are simply placeholders for real memory layouts that are replaced by real direct references during the parsing phase of the class loading process.
2.2.2 Structure of constant pool
Each constant in the constant pool is a table. These tables have a common feature, that is, the first bit of the table structure starts with a flag bit of type U1, which represents which constant type the current constant belongs to
The item type in the constant pool
| project | type | describe |
|---|---|---|
| CONSTANT_Utf8_info | 1 | The character string is utF-8 encoded |
| CONSTANT_Integer_info | 3 | Integer literal |
| CONSTANT_Float_info | 4 | Floating point literals |
| CONSTANT_Long_info | 5 | Long integer literals |
| CONSTANT_Double_info | 6 | A double – precision floating-point literal |
| CONSTANT_Class_info | 7 | Symbolic reference to a class or interface |
| CONSTANT_String_info | 8 | String type literals |
| CONSTANT_Fieldref_info | 9 | Symbolic reference to a field |
| CONSTANT_Methodref_info | 10 | Symbolic references to methods in a class |
| CONSTANT_InterfaceMethodref_info | 11 | Symbolic references to methods in the interface |
| CONSTANT_NameAndType_info | 12 | A partial symbolic reference to a field or method |
| CONSTANT_MethodHandle_info | 15 | Represents a method handle |
| CONSTANT_MethodType_info | 16 | Presentation method type |
| CONSTANT_Dynamic_info | 17 | Represents a dynamically computed constant |
| CONSTANT_InvkoeDynamic_info | 18 | Represents a dynamic method call point |
| CONSTANT_Module_info | 19 | Represents a module |
| CONSTANT_Package_info | 20 | Represents open or exported packages in a module |
Table information and table structure are as follows:
2.2.2.1 CONSTANT_Utf8_info
| type | mark | describe |
|---|---|---|
| tag | u1 | Value of 1 |
| length | u2 | Number of bytes in a UTF-8 encoded string |
| bytes | u1 | The length of the string is utF-8. The length is a total of length |
2.2.2.2 CONSTANT_Integer_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of 3 |
| bytes | u4 | Int values stored first in order of high order |
2.2.2.3 CONSTANT_Float_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of 4 |
| bytes | u4 | Float values stored first in order of magnitude |
2.2.2.4 CONSTANT_Long_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of 5 |
| bytes | u8 | The long value stored in front of the high order |
2.2.2.5 CONSTANT_Double_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of 6 |
| bytes | u8 | A double stored in front of the high order |
2.2.2.6 CONSTANT_Class_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of 7 |
| index | u2 | Index to a fully qualified named constant item |
2.2.2.7 CONSTANT_String_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of eight |
| index | u2 | Index to a string literal |
2.2.2.8 CONSTANT_Fieldref_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of 9 |
| index | u2 | Index entry to the class or interface descriptor CONSTANT_Class_info that declares the field |
| index | u2 | Index entry pointing to the field descriptor CONSTANT_NameAndType |
2.2.2.9 CONSTANT_Methodref_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of 10 |
| index | u2 | Index entry to the class or interface descriptor CONSTANT_Class that declares the method |
| index | u2 | Index entry pointing to the name and type descriptor CONSTANT_NameAndType |
2.2.2.10 CONSTANT_InterfaceMethodref_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of 11 |
| index | u2 | The index entry pointing to the interface descriptor CONSTANT_Class for the declared method |
| index | u2 | Index entry pointing to the name and type descriptor CONSTANT_NameAndType |
2.2.2.11 CONSTANT_NameAndType_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of 12 |
| index | u2 | Index to the constant entry of the field or method name |
| index | u2 | The index pointing to the constant entry of the field or method descriptor |
2.2.2.12 CONSTANT_MethodHandle_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of 15 |
| reference_kind | u1 | The value must be between 1 and 9 [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 for open eating |
2.2.2.13 CONSTANT_MethodType_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of 16 |
| 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 |
2.2.2.14 CONSTANT_Dynamic_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of 17 |
| bootstrap_method_attr_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 entry at that index must be a CONSTANT_NameAndType_info structure representing the method name and method descriptor |
2.2.2.15 CONSTANT_InvkoeDynamic_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of 18 |
| bootstrap_method_attr_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 entry at that index must be a CONSTANT_NameAndType_info structure representing the method name and method descriptor |
2.2.2.16 CONSTANT_Module_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of 19 |
| name_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 module name |
2.2.2.17 CONSTANT_Package_info
| project | type | describe |
|---|---|---|
| tag | u1 | A value of 19 |
| name_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 package name |
3. View the constant pool table
If we use a text editor to open a Class file, you might see something like this:
Other than the magic number CAFE BABE information can be quite difficult to read. Fortunately, Oracle provides a tool for analyzing Class files bytecode: Javap. How to use Javap:
javap -verbose xxxx.class
Copy the codeWrite a Java class Test as follows:
public class Test {
String test = "dafa";
String test1 = "soft";
String test3 = "dafasoft";
public Test(a) {}void Test(a) {
this.testFun();
}
public void testFun(a) {}public void testFun1(a) {}public void testFun2(a) {}public static void main(String[] args) {}}Copy the codeLet’s compile and use the Javap command to see what its constant pool table looks like. A screenshot of its constant pool table is shown below:
The Javap tool annotates the table for us automatically, but it actually makes sense if we look through the above table structure without comments. Here are two examples:
3.1. Example 1: String test3 = “dafasoft”;
This is a literal string constant defined in our Java class. How do we find it in the constant pool table? First of all, it is a Field. We first look for fieldrefs. There are three fieldrefs in the table above.
According to the table structure in section 2.2.2.8, the two values after Filedref are two indexes. An index to the class or interface descriptor CONSTANT_Class_info for the declared field and an index to the field descriptor CONSTANT_NameAndType, such as Fieldref at #7 with values #9.#40 and #9 with values #42. #42 The corresponding value string “com/dafasoft/test/ test”; The value of #40 is #14:#12, the value of #14 is the String test3, the value of #12 is the String “Ljava/lang/String; Together, the information channel, we can come to # 7 in the Fieldref is pointing to the com/dafasoft/test/test. The test3: Ljava/lang/String; A character reference to.
Up to now, we already know in the class has a reference point to com/dafasoft/test/test. The class variables test3, its type is String type, so how it is and its value “dafasoft” link? This is related to the initialization of the object, and the specific bytecode is in the method:
Notice lines 17 and 19 of the Code. LDC = ‘push int, float, String from constant pool to top of stack’ putfied = ‘assign field to instance of specified class’ After executing these two instructions, we are finished assigning the String variable test3.
3.2 Example 2: this.testfun ();
This statement is in the constructor Test(). To call a method, you need to know the reference to the method in the method area. Method references in the constant pool table are of the formCONSTANT_Methodref_infoIn this example, the constant pool table has index #8, which we will not expand. It is the same as the constant pool table in example 1com/dafasoft/test/Test.testFun:()VA symbolic reference to the test class in the com.dafasoft. Test packagevoid testFun()Methods. The call is in the bytecode of the Test method:
Look for the Code of the first line, the line instruction mean, call to character reference # 8 method, the character reference # 8 corresponding method is com/dafasoft/test/test testFun (V)
From this we can also conclude that a reference to a method is added to the constant pool only when a method call is made in a Java class. So where do the methods defined in this class go? For example, if the Test class has three methods, only testFun is called, and references to testFun1 and testFun2 do not appear in the constant pool table. So where are the implementations of these three methods? The answer is in the bytecode method table, which is a topic for another day.
There are 17 types in the constant pool table, and references to all 17 types can be derived in this way, so we won’t go through them all.