Objects
How is object stored in memory?
- The actual data/structure that is stored on the heap starts with what's commonly called object header.
- Header contains — a (compressed) class pointer
- Class pointer —> an internal data structure
- Internal data structure — defines layout of the class
- Layout of class — stored in a separate memory area called Metaspace (or Compressed Class space if Compressed OOPs are used).
- The pointer can be 4 or 8 bytes, depending on the architecture - even on 64-bit systems, it's usually 4 bytes due to the Compressed OOPs optimization.
Reachable Objects
An object is called reachable if it is reachable from a lang.java.jre.memory.objects.root (Private).
Objects which are directly or indirectly reachable from some other objects.
For example,
- P -> O
- P -> Q -> O
O is reachable from P in both cases.
Root Objects
An object is a root object if it is referenced by:
- A parameter on a call stack frame
- A local variable on the call stack frame
- A Static Variable in class
- Others like Class Loader or Java Native Interfaces (JNI) references.
Lifecycle
Box smallBox;
Unlike C++, the above statement won’t create an object. It will just create a reference variable. The reference variable has to be given an object.
Box smallBox= new Box();
Now smallBox contains address of an object of Box class. That is, it points to the object. However, the object itself has no name.