In the JVM, multiple threads share the heap and method area resources of a process, but each thread has its own program counter, virtual machine stack, and local method stack
What is a pile of
The Java heap is the largest chunk of memory managed by a Java VIRTUAL machine. The Java heap is an area of memory shared by all threads. It is created when the virtual machine is started and used to hold object instances.
The Java Heap is the primary area managed by the Garbage collector and is therefore also known as the Garbage Collected Heap. From the point of view of garbage collection, the Java heap can be subdivided into: new generation and old generation: Eden space, From Survivor, To Survivor space, etc. The purpose of further partitioning is to better reclaim memory, or to allocate memory faster.
Eden, S0 and S1 as shown in the figure above all belong to Cenozoic era, while Tentired belongs to old age. In most cases, the object will be allocated in Eden first. After a Cenozoic garbage collection, if the object is still alive, it will enter S0 or S1 and its age will be increased by 1(the initial age of the object becomes 1 after Eden ->Survivor). When it reaches a certain age (15 by default), it is promoted to the old age. The age threshold for the object to be promoted to the old age can be set by using -xx :MaxTenuringThreshold.
What is a method area
The method area, like the Java heap, is an area of memory shared by individual threads to store data such as class information that has been loaded by the virtual machine, constants, static variables, code compiled by the just-in-time compiler, and so on.
The runtime constant pool is also part of the method area. Known, when used in string assignment (not new), as well as the wrapper class Byte, Short, Integer, Long, Character, and Boolean value within the range 128 ~ 127
What is a virtual machine stack
Java memory can be roughly divided into Heap memory and Stack memory, where the Stack is now referred to as the virtual machine Stack, or the local variable scale part of the virtual machine Stack. (Actually, the Java virtual machine stack is made up of stack frames, and each stack frame has: local variable table, operand stack, dynamic link, method exit information.)
The local variable table mainly stores various data types known by the compiler (Boolean, byte, CHAR, short, int, float, long, double), object references (reference type, which is different from the object itself, may be a reference pointer to the object’s starting address. It may also point to a handle representing an object or other location associated with this object.
What is a local method stack
This function is very similar to that of the virtual machine stack, except that the virtual machine stack performs Java methods (that is, bytecode) services for the virtual machine, while the Native method stack serves Native methods used by the virtual machine. Integrate the HotSpot VIRTUAL machine with the Java virtual machine stack.
When a local method is executed, a stack frame is also created on the local method stack to hold the local variable table, operand stack, dynamic link, and exit information for the local method.
What is a program counter
A program counter is a small memory space that can be viewed as a line number indicator of the bytecode being executed by the current thread. The bytecode interpreter works by changing the value of this counter to select the next bytecode instruction to be executed. Branches, loops, jumps, exception handling, thread recovery and other functions rely on this counter to complete.
In addition, in order to restore the thread to the correct execution position after switching, each thread needs to have an independent program counter, which does not affect each other and is stored independently. We call this kind of memory area “thread private” memory.
Ref:
- This is probably the clearest article on the Java memory region