JVM runtime constant pool and direct memory

Runtime constant pool

The runtime constant pool is part of the method area .Class Except for the version of the class in the file 、 Field 、 Method 、 Interface and other description information , And the constant pool table .

Constant pool

A valid bytecode file contains the version information of the class 、 Field 、 Methods and interfaces , Also included is the constant pool , Contains a variety of literal quantities and pairs of types 、 Symbolic references to fields and methods .

Why constant pools are needed ？

One java Classes in the source file 、 Interface , After compiling, a bytecode file is generated . and Java The bytecode in needs data support , Usually this data is too large to be stored directly in bytecode , Another way , It can be stored in a constant pool , This bytecode contains a reference to the constant pool . The runtime constant pool is used in dynamic linking .

Data types stored in the constant pool

• Quantity is worth
• A string value
• Class reference
• Field reference
• Method reference

Runtime constant pool

• Runtime constant pool （Runtime Constant Pool） Is part of the method area .
• Constant pool table （Constant Pool Table） yes Class Part of the document , Used to store various literal quantities and symbol references generated by the compiler , This part will be stored in the runtime constant pool of the method area after the class is loaded .
• Runtime constant pool , be relative to Class Another important feature of the file constant pool is ： dynamic .

Direct memory

Direct memory is not part of the VM runtime data area , It's not the memory area defined in the virtual machine specification , But this part of memory is also frequently used . And it can also lead to OutOfMemoryError Error occurred .

Direct memory allocation is not affected by Java Pile limits , however , Since memory is limited by the total memory size of the machine and the addressing space of the processor .

Direct memory is Java Out of the pile 、 Memory range directly applied to the system . Access to direct memory is faster than Java Pile up , That is to say, the read-write performance is high .