Garbage Collection

• Garbage collection is automatic memory management by JVM.

• If a Java object is unreachable (i.e. not accessible through any reference), then it is automatically released by the garbage collector.

• An object becomes eligible for GC in one of the following cases:

  • Nullify the reference

    MyClass obj = new MyClass();
    obj = null;
    

  • Reassign the reference

    MyClass obj = new MyClass();
    obj = new MyClass();
    

  • Object created locally in a method

    void method() {
        MyClass obj = new MyClass();
        // ...
    }
    

• GC is a background thread in JVM that runs periodically and reclaims memory of unreferenced objects.

• Before an object is destroyed, its finalize() method is invoked (if present).

• One should override this method if the object holds any resource to be released explicitly, e.g., file close, database connection, etc.

class MyClass {
    private Connection con;
    public MyClass() throws Exception {
        con = DriverManager.getConnection("url", "username", "password");
    }
    @Override
    public void finalize() {
        try {
            if(con != null)
                con.close();
        } catch(Exception e) { }
    }
}
class Main {
	public static void method() throws Exception {
		MyClass my = new MyClass();
		my = null;
		System.gc(); // request GC
	}
	// ...
}

• GC can be requested (not forced) by one of the following:

System.gc();
Runtime.getRuntime().gc();

• JVM GC internally uses the Mark and Compact algorithm.
• GC Internals

JVM Architecture

Java Compilation Process

Hello.java --> Java Compiler --> Hello.class
javac Hello.java

• The Java compiler converts Java code into bytecode.

Bytecode

• Bytecode consists of machine-level instructions that can be executed by the JVM.
  • Instruction = Op code + Operands
    • e.g. iadd op1, op2
• Each instruction in byte-code is of 1 byte.

.class --> JVM --> Windows (x86)
.class --> JVM --> Linux (ARM)