Memory Model

The Java Virtual Machine defines various run-time data areas that are used during execution of a program.

Created during the virtual machine startup
Allocated for all class instances and arrays
A fixed or dynamic size: Programmer control the max and min size, be careful OutOfMemory.
The memory does not need to be contiguous
Shared by threads

Runtime Constant Pool
- Numerical constant
- Class attribute reference
- Method reference
Field Data: the name, type…of each class
Method Data: the name, return value type, parameter list… of each method
Method Code: bytecode, local variable table… of each method
May be of a fixed size or may be expanded
The memory does not need to be contiguous

When the thread starts, a runtime stack will be assigned to the thread.
The stack stores frames. A frame is used to store data and partial results and to return values for methods, and dispatch exceptions.
May be of a fixed size or may be expanded
The memory does not need to be contiguous

Support native methods
- Methods are written in a language other than the Java programming language such as C/C++.
  
  To load native method, we’ll use the System.loadLibrary.
  
  Place the call in a static block so that it is available in our class:
```
  public class DateTimeUtils {
        
  	public native String getSystemTime();
        
  	static {
         System.loadLibrary("nativedatetimeutils");
    }
  }
```
Allocated per each thread when each thread is created.
Java Virtual Machine implementations that cannot load native methods and that do not themselves rely on stacks need not supply native method stacks.
Size can be either fixed or dynamic.

Each of the JVM threads has its own pc register
Save the non-native code address of the current execution (memory address in the method area) and if the method is native, the value of the PC register is undefined.