참조(Reference)와 GC(Garbage Collection)의 관계

참조(Reference)와 GC(Garbage Collection)의 관계

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1. Strong Reference

public class JavaTest {
    public static void main(String[] args) {

        MyClass obj = new MyClass();

        obj = null;
        // "obj" object is no longer referencing to the instance.
        // So the "MyClass" type object is now available for garbage collection.
    }
}

class MyClass {
}

// Java program to illustrate Strong reference 
class MyClass {
    //Code..
}

public class JavaTest {
    public static void main(String[] args) {
        //Strong Reference - by default
        MyClass obj = new MyClass();

        //Now, object to which 'obj' was pointing earlier is
        //eligible for garbage collection.
        obj = null;
    }
} 

Strong Reference

 

 

 

 

2. Soft Reference

//Code to illustrate Soft reference
import java.lang.ref.SoftReference;

class MyClass {
    //code..
    public void print()
    {
        System.out.println("James Kim");
    }
}

public class JavaTest {
    public static void main(String[] args) {
        // Strong Reference
        MyClass obj = new MyClass();
        obj.print();

        // Creating Soft Reference to MyClass-type object to which 'obj'
        // is also pointing.
        SoftReference<MyClass> softRef = new SoftReference<MyClass>(obj);

        // Now, MyClass-type object to which 'obj' was pointing earlier
        // is available for garbage collection.
        // But, it will be garbage collected only when JVM needs memory.
        obj = null;

        // You can retrieve back the object which
        // has been weakly referenced.
        // It successfully calls the method.
        obj = softRef.get();

        obj.print();
    }
} 

대상 객체를 참조하는 경우가 SoftReference 객체만 존재하는 경우 GC의 대상이 됩니다.
단, JVM의 메모리가 부족한 경우에만 Heap 영역에서 제거되고 메모리가 부족하지 않다면 굳이 제거하지 않습니다.

 

실행 결과

Soft Reference

 

 

 

 

3. Weak Reference

// Java Code to illustrate Weak reference
import java.lang.ref.WeakReference;

class MyClass {
    // code
    public void print() {
        System.out.println("James Kim");
    }
}

public class JavaTest {
    public static void main(String[] args) {
        // Strong Reference
        MyClass obj = new MyClass();
        obj.print();

        // Creating Weak Reference to MyClass-type object to which 'obj'
        // is also pointing.
        WeakReference<MyClass> weakRef = new WeakReference<MyClass>(obj);

        // Now, MyClass-type object to which 'obj' was pointing earlier
        // is available for garbage collection.
        obj = null;

        // You can retrieve back the object which
        // has been weakly referenced.
        // It successfully calls the method.
        obj = weakRef.get();

        obj.print();
    }
}

대상 객체를 참조하는 경우가 WeakReferences 객체만 존재하는 경우 GC의 대상이 됩니다.
다음 GC 실행 시 무조건 Heap 메모리에서 제거됩니다.

 

실행 결과

Weak Reference

 

 

 

 

 

4. Phantom Reference

//Code to illustrate Phantom reference
import java.lang.ref.*;

class MyClass {
    // code
    public void print() {
        System.out.println("James Kim");
    }
}

public class JavaTest {
    public static void main(String[] args) {
        // Strong Reference
        MyClass obj = new MyClass();
        obj.print();

        // Creating reference queue
        ReferenceQueue<MyClass> refQueue = new ReferenceQueue<MyClass>();

        // Creating Phantom Reference to MyClass-type object to which 'obj'
        // is also pointing.
        PhantomReference<MyClass> phantomRef = new PhantomReference<MyClass>(obj, refQueue);

        // Now, MyClass-type object to which 'obj' was pointing
        // earlier is available for garbage collection.
        // But, this object is kept in 'refQueue' before
        // removing it from the memory.
        obj = null;

        // It always returns null.
        obj = phantomRef.get();

        // It shows NullPointerException.
        obj.print();
    }
}

실행 결과

 

 

 

 

 

 

참고.

www.geeksforgeeks.org/types-references-java/

Types of References in Java - GeeksforGeeks

 

lion-king.tistory.com/entry/Java-참조-유형-Strong-Reference-Soft-Reference-Weak-Reference-Phantom-References

(Java) 참조 유형 (Strong Reference/ Soft Reference/ Weak Reference/ Phantom References)

 

neverfear.org/blog/view/150/Strong_Soft_Weak_and_Phantom_References_Java

NEVERFEAR.org - Strong, Soft, Weak and Phantom References (Java)

 

madplay.github.io/post/java-garbage-collection-and-java-reference

자바 레퍼런스와 가비지 컬렉션(Java Reference & Garbage Collection)