Runtime data areas

Learn stack, heap and metaspace.

Runtime data areas

The JVM stores runtime data in separate data areas. The main ones are the heap (objects), the stack (method frames), Metaspace (class metadata) and the program counter (PC register).

The heap holds all class instances and arrays; it is shared and managed by the GC. The stack is per-thread; each method call pushes a frame. The frame holds local variables and the operand stack.

Metaspace holds class and method metadata. The PC register tracks which instruction the thread is running.

The stack frame

Each method call pushes a frame onto the stack. The frame contains three components: local variables, the operand stack and the method exit section.

When the method returns, the frame is popped from the stack. The stack is per-thread; therefore two threads' local variables do not mix. Stack overflow occurs in deep recursion.

Exercise Print the thread name

Define a Main class. In main, print the value of Thread.currentThread().getName() with System.out.println. (The main thread's name is 'main'.)

Show example solution

public class Main {
    public static void main(String[] args) {
        System.out.println(Thread.currentThread().getName());
    }
}

Test yourself

Complete the Thread call

Thread t = Thread.____();
System.out.println(t.getName());

Show answer

Thread t = Thread.currentThread();
System.out.println(t.getName());

What kind of data does the heap hold?
- Local variables
- Class instances and arrays
- Instructions
- Thread states
Show answer

Class instances and arrays — The heap holds all objects and arrays; it is managed by the GC.
Match the data areas

Heap

Objects and arrays
Stack

Method frames
Metaspace

Class metadata
PC register

Current instruction

Next lesson: Summary - JVM internals

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