Java Virtual Machine Learning - Object Access

Object access will involve three memory areas: Java stack, Java heap, and method area.

Such as the following code:

Object objectRef = new Object();

Assuming this code appears in the method body, the "Object objectRef" part will be reflected in the local variables of the Java stack as a reference type Data appears. The "new Object()" part will be reflected in the Java heap, forming a structured memory that stores all instance data values ​​of the Object type. The length of this memory depends on the specific type and the object memory layout implemented by the virtual machine. It is not fixed. In addition, the Java heap must also include address information that can find this object type data (such as object type, parent class, implemented interface, method, etc.). These data types are stored in the method area.

The reference type only stipulates a reference address pointing to the object in the Java virtual machine specification. It does not define the way in which this reference should be located and access the object location in the Java heap. Therefore, different virtual machines The access methods implemented by the machine may be different. There are two mainstream methods: using handles and direct pointers.

                  Handle access method: A piece of memory will be divided into the Java heap as a handle pool. What is stored in the reference is the handle address of the object, and the handle contains the object instance data and type data respective specific address information.

Pointer access method: What is directly stored in the reference variable is the address of the object, and part of the java heap object stores the object instance data, and the other part stores Object type data.

These two methods of accessing objects have their own advantages. The biggest advantage of using the handle access method is that the stable handle address is stored in the reference. When the object is moved, Only the instance data pointer in the handle needs to be changed, and the reference does not need to be changed. The biggest advantage of using the pointer access method is that it is fast. It saves the time overhead of pointer positioning. As far as the virtual machine is concerned, it uses the second method (direct pointer access).

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