Hướng dẫn và ví dụ Java SoftReference

package org.o7planning.beans;
 
public class AnyObject {
    private String val;
    public AnyObject(String val) {
        this.val = val;
    }
    public String getVal() {
        return this.val;
    }
    @Override
    protected void finalize() throws Throwable {
        super.finalize();
        System.out.println("I am being removed from memory");
    }
}

package org.o7planning.softreference.ex;

import java.lang.ref.SoftReference;
import java.util.ArrayList;
import java.util.List;

import org.o7planning.beans.AnyObject;

public class SoftReference_obj_ex1 {

    public static void main(String[] args) throws InterruptedException {
        // Create myAnyObject reference points to AnyObject("Obj1").
        AnyObject myAnyObject = new AnyObject("Obj1");

        // Create SoftReference object
        SoftReference<AnyObject> softRef = new SoftReference<AnyObject>(myAnyObject);

        System.out.println("softRef.get(): " + softRef.get());

        List<String> list= new ArrayList<String>();

        int i = 0;
        String s = "";
        while (true) {
            AnyObject innerObject = softRef.get();
            if (innerObject == null) {
                System.out.println("Inner object is removed by Garbage Collector");
                System.out.println("softRef.get(): " + innerObject);
                break;
            }
            i++;
            //
            s = s + " String " + i; // Throw OutOfMemoryError
            list.add(s);
            System.out.println("Create new String: " + i);
        }
    }
}

-XX:SoftRefLRUPolicyMSPerMB=1000

package org.o7planning.softreference.ex;

import java.lang.ref.SoftReference;
import java.util.ArrayList;
import java.util.List;

import org.o7planning.beans.AnyObject;

public class SoftReference_obj_ex2 {

    public static void main(String[] args) throws InterruptedException {
        // A String has size of 1Mb.
        String oneMbString = create1MbString();

        // Create myAnyObject (~2 Mb in HEAP).
        AnyObject myAnyObject = new AnyObject(oneMbString + oneMbString);

        // Create SoftReference object
        SoftReference<AnyObject> softRef = new SoftReference<AnyObject>(myAnyObject);

        System.out.println("softRef.get(): " + softRef.get());
        myAnyObject = null;

        List<String> list = new ArrayList<String>();

        int i = 0;

        while (true) {
            i++;
            long freeMemoryInBytes = Runtime.getRuntime().freeMemory(); // in bytes
            long freeMemoryInMbs = freeMemoryInBytes / (1024 * 1024);
            System.out.println("Free memory in Mb: " + freeMemoryInMbs);

            //
            if (freeMemoryInMbs <= 10) {
                Thread.sleep(1200);
            }
            if (freeMemoryInMbs <= 2) {
                System.out.println("Done!");
                break;
            }
            System.out.println(" >> Create new String");
            String s = oneMbString + " - " + i;
            list.add(s);
        }
    }

    // Create a String has the size of 1MB.
    // 1MB = 1024KB = 1024x1024 bytes = (2^10)*(2^10) bytes = 2^20 bytes.
    private static String create1MbString() {
        String s = "A"; // 2 bytes
        for (int i = 0; i < 20; i++) {
            s = s + s;
        }
        return s;
    }
}

SoftReference(T referent)

SoftReference(T referent, ReferenceQueue<? super T> queue)

// Methods inherited from parent.
public T get()  
public void clear()   
public boolean isEnqueued()   
public boolean enqueue()

SoftReference(T innerObject, ReferenceQueue<? super T> queue)

package org.o7planning.softreference.cache.ex;

import java.lang.ref.SoftReference;
import java.util.HashMap;
import java.util.Map;

public class ImageCache {
    private static final ImageCache instance = new ImageCache();

    private Map<String, SoftReference<byte[]>> cacheMap = new HashMap<>();

    private ImageCache() {
    }

    public static ImageCache getInstance() {
        return instance;
    }

    public byte[] getImageData(String imagePath) {
        SoftReference<byte[]> value = this.cacheMap.get(imagePath);
        
        byte[] data = null;
        if(value == null || (data = value.get()) == null)  {
            data = this.readImageFromDisk(imagePath);
            this.cacheMap.put(imagePath, new SoftReference<byte[]>(data));
            System.out.println(">> Load data from disk: " + imagePath);
        } else  {  
            System.out.println("   Found data in cache: " + imagePath);
        }  
        return data;
    }

    private byte[] readImageFromDisk(String imagePath) {
        // Read from disk..
        return new byte[3];
    }
}

package org.o7planning.softreference.cache.ex;

public class ImageCacheTest {

    public static void main(String[] args) throws InterruptedException {
        String[] imagePaths = new String[] { //
                "image1.png", //
                "image1.png", //
                "image2.png", //
                "image2.png", //
                "image1.png", //
                "image3.png", //
                "image3.png", //
                "image1.png", //
        };

        ImageCache cache = ImageCache.getInstance();

        for (int i = 0; i < imagePaths.length; i++) {
            byte[] data = cache.getImageData(imagePaths[i]);
            Thread.sleep(500);
        }

        System.out.println("Done!");
    }
}