Android网络图片异步加载

2021-07-05 04:16:27

前言

其实很幸运，入职一周之后就能跟着两个师兄做android开发，师兄都是大神，身为小白的我只能多多学习，多多努力。最近一段时间都忙的没机会总结，今天刚完成了android客户端图片异步加载的类，这里记录一下（ps：其实我这里都是参考网上开源实现）


原理

在ListView或者GridView中加载图片的原理基本都是一样的：
  1. 先从内存缓存中获取，取到则返回，取不到进行下一步
  2. 从文件缓存中获取，取到则返回并更新到内存缓存，取不到则进行进行下一步
  3. 从网络上下载图片，并更新内存缓存和文件缓存

流程图如下：

Android网络图片异步加载



同时，要注意线程的数量。一般在listview中加载图片，大家都是开启新的线程去加载，但是当快速滑动时，很容易造成OOM，因此需要控制线程数量。我们可以通过线程池控制线程的数量，具体线程池的大小还需要根据处理器的情况和业务情况自行判断

建立线程池的方法如下：

ExecutorService executorService = Executors.newFixedThreadPool(5); // 5是可变的


文件缓存类

import java.io.File;

import android.content.Context;

public class FileCache {
	private static final String DIR_NAME = "your_dir";
	private File cacheDir;

	public FileCache(Context context) {
		// Find the directory to save cached images
		if (android.os.Environment.getExternalStorageState().equals(
				android.os.Environment.MEDIA_MOUNTED)) {
			cacheDir = new File(
					android.os.Environment.getExternalStorageDirectory(),
					DIR_NAME);
		} else {
			cacheDir = context.getCacheDir();
		}

		if (!cacheDir.exists()) {
			cacheDir.mkdirs();
		}
	}

	public File getFile(String url) {
		// Identify images by url‘s hash code
		String filename = String.valueOf(url.hashCode());

		File f = new File(cacheDir, filename);

		return f;
	}

	public void clear() {
		File[] files = cacheDir.listFiles();
		if (files == null) {
			return;
		} else {
			for (File f : files) {
				f.delete();
			}
		}
	}
}


内存缓存类

这里使用了软引用，Map<String, SoftReference<Bitmap>> cache，可以google一下软引用的机制，简单的说：实现了map，同时当内存紧张时可以被回收，不会造成内存泄露

import java.lang.ref.SoftReference;
import java.util.Collections;
import java.util.LinkedHashMap;
import java.util.Map;

import android.graphics.Bitmap;

public class MemoryCache {
	private Map<String, SoftReference<Bitmap>> cache = Collections
			.synchronizedMap(new LinkedHashMap<String, SoftReference<Bitmap>>(
					10, 1.5f, true));

	public Bitmap get(String id) {
		if (!cache.containsKey(id)) {
			return null;
		}

		SoftReference<Bitmap> ref = cache.get(id);

		return ref.get();
	}

	public void put(String id, Bitmap bitmap) {
		cache.put(id, new SoftReference<Bitmap>(bitmap));
	}

	public void clear() {
		cache.clear();
	}
}


图片加载类

package io.yunos.bbs;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.Collections;
import java.util.Map;
import java.util.WeakHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Handler;
import android.widget.ImageView;

public class ImageLoader {
	/**
	 * Network time out
	 */
	private static final int TIME_OUT = 30000;
	/**
	 * Default picture resource
	 */
	private static final int DEFAULT_BG = R.drawable.plate_list_head_bg;

	/**
	 * Thread pool number
	 */
	private static final int THREAD_NUM = 5;

	/**
	 * Memory image cache
	 */
	MemoryCache memoryCache = new MemoryCache();

	/**
	 * File image cache
	 */
	FileCache fileCache;

	/**
	 * Judge image view if it is reuse
	 */
	private Map<ImageView, String> imageViews = Collections
			.synchronizedMap(new WeakHashMap<ImageView, String>());

	/**
	 * Thread pool
	 */
	ExecutorService executorService;

	/**
	 * Handler to display images in UI thread
	 */
	Handler handler = new Handler();

	public ImageLoader(Context context) {
		fileCache = new FileCache(context);
		executorService = Executors.newFixedThreadPool(THREAD_NUM);
	}

	public void disPlayImage(String url, ImageView imageView) {
		imageViews.put(imageView, url);
		Bitmap bitmap = memoryCache.get(url);
		if (bitmap != null) {
			// Display image from Memory cache
			imageView.setImageBitmap(bitmap);
		} else {
			// Display image from File cache or Network
			queuePhoto(url, imageView);
		}
	}

	private void queuePhoto(String url, ImageView imageView) {
		PhotoToLoad photoToLoad = new PhotoToLoad(url, imageView);
		executorService.submit(new PhotosLoader(photoToLoad));
	}

	private Bitmap getBitmap(String url) {
		File f = fileCache.getFile(url);

		// From File cache
		Bitmap bmp = decodeFile(f);
		if (bmp != null) {
			return bmp;
		}

		// From Network
		try {
			Bitmap bitmap = null;
			URL imageUrl = new URL(url);
			HttpURLConnection conn = (HttpURLConnection) imageUrl
					.openConnection();
			conn.setConnectTimeout(TIME_OUT);
			conn.setReadTimeout(TIME_OUT);
			conn.setInstanceFollowRedirects(true);
			InputStream is = conn.getInputStream();
			OutputStream os = new FileOutputStream(f);
			copyStream(is, os);
			os.close();
			conn.disconnect();
			bitmap = decodeFile(f);
			return bitmap;
		} catch (Throwable ex) {
			if (ex instanceof OutOfMemoryError) {
				clearCache();
			}
			return null;
		}

	}

	private void copyStream(InputStream is, OutputStream os) {
		int buffer_size = 1024;

		try {
			byte[] bytes = new byte[buffer_size];
			while (true) {
				int count = is.read(bytes, 0, buffer_size);
				if (count == -1) {
					break;
				}
				os.write(bytes, 0, count);
			}

		} catch (Exception e) {

		}
	}

	private Bitmap decodeFile(File f) {
		try {
			// TODO:Compress image size
			FileInputStream fileInputStream = new FileInputStream(f);
			Bitmap bitmap = BitmapFactory.decodeStream(fileInputStream);
			return bitmap;

		} catch (FileNotFoundException e) {
			return null;
		}
	}

	private void clearCache() {
		memoryCache.clear();
		fileCache.clear();
	}

	/**
	 * Task for the queue
	 * 
	 * @author zhengyi.wzy
	 * 
	 */
	private class PhotoToLoad {
		public String url;
		public ImageView imageView;

		public PhotoToLoad(String url, ImageView imageView) {
			this.url = url;
			this.imageView = imageView;
		}
	}

	/**
	 * Asynchronous to load picture
	 * 
	 * @author zhengyi.wzy
	 * 
	 */
	class PhotosLoader implements Runnable {
		PhotoToLoad photoToLoad;

		public PhotosLoader(PhotoToLoad photoToLoad) {
			this.photoToLoad = photoToLoad;
		}

		private boolean imageViewReused(PhotoToLoad photoToLoad) {
			String tag = imageViews.get(photoToLoad.imageView);
			if (tag == null || !tag.equals(photoToLoad.url)) {
				return true;
			}

			return false;
		}

		@Override
		public void run() {
			// Abort current thread if Image View reused
			if (imageViewReused(photoToLoad)) {
				return;
			}

			Bitmap bitmap = getBitmap(photoToLoad.url);

			// Update Memory
			memoryCache.put(photoToLoad.url, bitmap);

			if (imageViewReused(photoToLoad)) {
				return;
			}

			// Don‘t change UI in children thread
			BitmapDisplayer bd = new BitmapDisplayer(bitmap, photoToLoad);
			handler.post(bd);
		}

		class BitmapDisplayer implements Runnable {
			Bitmap bitmap;
			PhotoToLoad photoToLoad;

			public BitmapDisplayer(Bitmap bitmap, PhotoToLoad photoToLoad) {
				this.bitmap = bitmap;
				this.photoToLoad = photoToLoad;
			}

			@Override
			public void run() {
				if (imageViewReused(photoToLoad)) {
					return;
				}

				if (bitmap != null) {
					photoToLoad.imageView.setImageBitmap(bitmap);
				} else {
					photoToLoad.imageView.setImageResource(DEFAULT_BG);
				}
			}

		}
	}
}


调用方法

ImageLoader imageLoader = new ImageLoader(context);
imageLoader.disPlayImage(imageUrl, imageView);


小结

最近还好，终于开始忙起来了，我还是喜欢紧张一点的节奏，能学到更多的东西




Android网络图片异步加载,布布扣,bubuko.com

Android网络图片异步加载

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