先上效果图：

如图，android默认也有Gallery,很多软件在调用时，都是使用自己的Gallery，一方面好维护，另外一方面可以做优化。要做成以上样式，图片加载类起至关重要，一不小心，就好OOM, 下面这个类就是做Gallery的核心。

package com.example.gallery.utils;

import java.lang.reflect.Field;
import java.util.LinkedList;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.BitmapFactory.Options;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.support.v4.util.LruCache;
import android.util.DisplayMetrics;
import android.view.ViewGroup.LayoutParams;
import android.widget.ImageView;

public class ImageLoader {

    public static ImageLoader mInstance;
    private LruCache<String, Bitmap> mLruCache;
    private ExecutorService mThreadPool;
    private static final int DEFAULT_THREAD_POOL_SIZE = 1;
    private Type mType = Type.LIFO; //队列的调度方式
    private LinkedList<Runnable> mTaskQueue;  //任务队列，可以从头部和尾部取对象，链表不用连续的内存
    private Thread mPoolThread; //后台轮询线程
    private Handler mPoolThreadHandler;
    private Handler mUIHandler; //UI线程
    private Semaphore mSemaphorePoolThreadHandler = new Semaphore(0); //信号量用来同步,默认申请0
    private Semaphore mSemaphoreThreadPool;

    public enum Type {
        FIFO,LIFO;
    }
    private ImageLoader(int threadCount, Type type) {
        init(threadCount, type);
    }

    private void init(int threadCount, Type type) {
        //后台轮询线程
        mPoolThread = new Thread() {
            @Override
            public void run() {
                Looper.prepare();
                mPoolThreadHandler = new Handler(){
                    public void handleMessage(android.os.Message msg) {
                        //线程池去取出一个任务进行执行
                        mThreadPool.execute(getTask());
                        try {
                            mSemaphoreThreadPool.acquire(); //阻塞住
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    };
                };
                mSemaphorePoolThreadHandler.release();//释放信号量
                Looper.loop();
            };
        };
        mPoolThread.start();
        //获取我们应用的最大可用内存
        int maxMemory = (int) Runtime.getRuntime().maxMemory();
        int cacheMemory = maxMemory / 8;
        mLruCache = new LruCache<String, Bitmap>(cacheMemory){
            protected int sizeOf(String key, Bitmap value) {

                return value.getRowBytes() * value.getHeight();
            };// 每行的字节数*高度
        };
        //创建线程池
        mThreadPool = Executors.newFixedThreadPool(threadCount);
        mTaskQueue = new LinkedList<Runnable>();
        mType = type;
        mSemaphoreThreadPool = new Semaphore(threadCount);
    }

    public static ImageLoader getInstance(int size, Type type) {
        if(mInstance == null) {
            //效率的提升，如果多个线程进入时
            synchronized (ImageLoader.class) {
                if(mInstance == null) { // 每次都new ,会产生多个对象
                    mInstance = new ImageLoader(DEFAULT_THREAD_POOL_SIZE, Type.LIFO);
                }
            }
        }
        return mInstance;
    }

    public void loadImage(final String path, final ImageView image) {
        image.setTag(path);//防止多次复用
        if(mUIHandler == null) {
            mUIHandler = new Handler() {
                public void handleMessage(android.os.Message msg) {
                    //获取得到图片，为image回调设置图片
                    ImageBeanHolder holder = (ImageBeanHolder) msg.obj;
                    Bitmap bm = holder.bitmap;
                    ImageView imageview = holder.image;
                    String path = holder.path;
                    //将path与getTag存储路径进行比较
                    if(imageview.getTag().toString().equals(path)) {
                        imageview.setImageBitmap(bm);
                    }

                };
            };
        }
        Bitmap bm = getBitmapFromLruCache(path);
        if (bm != null) {
            refreshBitmap(path, image, bm);
        } else {
            addTask(new Runnable() {

                @Override
                public void run() {
                    //加载图片
                    //图片的压缩
                    //1.获得图片需要显示的大小
                    ImageSize imageViewSize = getImageViewSize(image);
                    //2.压缩图片
                    Bitmap bm = decodeSampleBitmapFromPath(path, imageViewSize.width, imageViewSize.height);
                    //3.把图片加入到缓存
                    addBitmapToLruCache(path,bm);
                    //4.进行回调
                    refreshBitmap(path, image, bm);
                    mSemaphoreThreadPool.release(); //任务完成，就施放信号量
                }
            });
        }
    }

    //从任务队列中取任务
    private Runnable getTask() {
        if(mType == Type.FIFO) {
            return mTaskQueue.removeFirst();
        } else if(mType == Type.LIFO) {
            return mTaskQueue.removeLast();
        }
        return null;
    }

    private void refreshBitmap(final String path, final ImageView image, Bitmap bm) {
        Message msg = Message.obtain();
        ImageBeanHolder holder = new ImageBeanHolder();
        holder.bitmap = bm;
        holder.image = image;
        holder.path = path;
        msg.obj = holder;
        mUIHandler.sendMessage(msg);
    }

    //将图片加到LruCache
    private void addBitmapToLruCache(String path, Bitmap bm) {
        if(getBitmapFromLruCache(path) == null) {
            if(bm != null) {
                mLruCache.put(path, bm);
            }
        }
    }

    private ImageSize getImageViewSize(ImageView image) {
        ImageSize imageSize = new ImageSize();
        DisplayMetrics displayMetrics = image.getContext().getResources().getDisplayMetrics();
        LayoutParams layoutParams = image.getLayoutParams();
        int width = image.getWidth(); //获取实际宽度
//        int width = getImageViewFiledValue(image, "mMaxWidth");
        //layoutParams.width == LayoutParams.WRAP_CONTENT ? 0 :
        if(width <= 0) { //wrap_content -1 fill_parent -2
            width = layoutParams.width; //获取image在layout中声明的宽度
        }
        if(width <= 0) {
            width = getImageViewFiledValue(image, "mMaxWidth"); //检查最大值
        }
        if(width <= 0) {
            width = displayMetrics.widthPixels;//为屏幕的宽度
        }

        int height = image.getHeight(); //获取实际高度
        //layoutParams.width == LayoutParams.WRAP_CONTENT ? 0 :
        if(height <= 0) { //wrap_content -1 fill_parent -2
            height = layoutParams.height; //获取image在layout中声明的高度
        }
        if(height <= 0) {
            height = getImageViewFiledValue(image, "mMaxHeight"); //检查最大值
        }
        if(height <= 0) {
            height = displayMetrics.heightPixels;//为屏幕的高度
        }
        imageSize.height = height;
        imageSize.width = width;
        return imageSize;
    }

    //为什么用反射，因为检查最大值是API 16才能用，兼容API 8 时，就用反射
    private static int getImageViewFiledValue(Object object,String fieldName) {
        int value = 0;
        try {
            Field field = ImageView.class.getDeclaredField(fieldName);
            field.setAccessible(true);
            int fieldValue = field.getInt(object);
            if(fieldValue > 0 && fieldValue < Integer.MAX_VALUE) {
                value = fieldValue;
            }
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }
        return value;
    }

    //根据显示的宽和高对图片进行压缩
    private Bitmap decodeSampleBitmapFromPath(String path, int width, int height) {

        //获取图片的宽和高，并不把图片加载到内存中
        BitmapFactory.Options  options = new BitmapFactory.Options();

        options.inJustDecodeBounds = true;
        BitmapFactory.decodeFile(path, options);
        options.inSampleSize = caculateInSampleSize(options, width, height);

        options.inJustDecodeBounds = false; //把图片加载到内存中
        Bitmap bitmap = BitmapFactory.decodeFile(path, options);//已经进行压缩
        return bitmap;
    }

    private int caculateInSampleSize(Options options, int reqWidth, int reqHeight) {
        int width = options.outWidth;
        int height = options.outHeight;
        int inSampleSize = 1;
        if(width > reqWidth || height > reqHeight) {
            int widthRadio = Math.round(1.0f * width / reqWidth);
            int heightRadio = Math.round(1.0f * width / reqHeight);
            inSampleSize = Math.max(widthRadio, heightRadio);
        }
        return inSampleSize;
    }

    private synchronized void addTask(Runnable runnable) {
        mTaskQueue.add(runnable);
//        if(mPoolThreadHandler == null)
//            wait();
        try {
            if(mSemaphorePoolThreadHandler == null)
            mSemaphorePoolThreadHandler.acquire();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        mPoolThreadHandler.sendEmptyMessage(0x110); //发送通知
    }

    private Bitmap getBitmapFromLruCache(String key) {
        return mLruCache.get(key);
    }

    private class ImageSize {
        int width;
        int height;
    }

    private class ImageBeanHolder{ //防止错乱
        Bitmap bitmap;
        ImageView image;
        String path;
    }
}