使用 HttpClient 下载图片

使用 JDK 自带的 UrlConnection 也能访问网络资源，但是要处理 SSL，Cookie，Proxy 等时编码就会很麻烦，Apache HttpClient 的功能非常强大，封装了很多操作，使用简单，能很方便的访问网络资源，其示例代码中有很多有用的例子，下载地址为 http://hc.apache.org/downloads.cgi

├── ClientAbortMethod.java
├── ClientAuthentication.java
├── ClientChunkEncodedPost.java
├── ClientConfiguration.java
├── ClientConnectionRelease.java
├── ClientCustomContext.java
├── ClientCustomPublicSuffixList.java
├── ClientCustomSSL.java
├── ClientEvictExpiredConnections.java
├── ClientExecuteProxy.java
├── ClientExecuteSOCKS.java
├── ClientFormLogin.java
├── ClientMultiThreadedExecution.java
├── ClientPreemptiveBasicAuthentication.java
├── ClientPreemptiveDigestAuthentication.java
├── ClientProxyAuthentication.java
├── ClientWithRequestFuture.java
├── ClientWithResponseHandler.java
├── ProxyTunnelDemo.java
├── QuickStart.java

还有 FluentApi 的例子

├── FluentAsync.java
├── FluentExecutor.java
├── FluentQuickStart.java
├── FluentRequests.java
└── FluentResponseHandling.java

下面简单的介绍一下使用 HttpClient 下载一幅图片，FluentApi 使用更加简洁，但是功能相对较少。

Gradle 依赖

dependencies {
    compile 'org.apache.httpcomponents:httpclient:4.5.2'
    compile 'org.apache.httpcomponents:fluent-hc:4.5.2'
}

使用 FluentApi 下载文件

The fluent API relieves the user from having to deal with manual deallocation of system resources at the cost of having to buffer response content in memory in some cases.

使用 FluentApi 不需要手动的缓存 response 数据到内存和释放相应的资源，FluentApi 已经帮我们处理好了。

import org.apache.http.client.fluent.Request;

import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;

public class DownloadUtil {
    /**
     * 使用 HttpClient 的 FluentApi 下载文件
     * @param url 文件的 url
     * @param localPath 本地存储路径
     * @throws IOException 如果 url 的文件找不到，超时等会抛出异常
     */
    public static void downloadFile(String url, String localPath) throws IOException {
        byte[] content = Request.Get(url).connectTimeout(5000).socketTimeout(5000).execute().returnContent().asBytes();
        BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream(new File(localPath)));
        out.write(content);
        out.flush();
        out.close();
    }

    public static void main(String[] args) throws IOException {
        downloadFile("http://xtuer.github.io/img/dog.png", "/Users/Biao/Desktop/a.png"); // 下载图片
    }
}

使用普通的 Api 下载文件

同样是下载文件，相比之下，代码复杂了不少

import org.apache.http.HttpEntity;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;

import java.io.*;

public class CommonDownloadUtil {
    public static void downloadFile(String url, String localPath) throws IOException {
        CloseableHttpClient httpclient = HttpClients.createDefault();
        try {
            HttpGet httpget = new HttpGet(url);

            System.out.println("Executing request " + httpget.getRequestLine());
            CloseableHttpResponse response = httpclient.execute(httpget);
            try {
                System.out.println("----------------------------------------");
                System.out.println(response.getStatusLine());

                // Get hold of the response entity
                HttpEntity entity = response.getEntity();

                // If the response does not enclose an entity, there is no need
                // to bother about connection release
                if (entity != null) {
                    InputStream in = entity.getContent();
                    try {
                        // do something useful with the response
                        byte[] buffer = new byte[1024];
                        BufferedInputStream bufferedIn = new BufferedInputStream(in);
                        int len = 0;

                        FileOutputStream fileOutStream = new FileOutputStream(new File(localPath));
                        BufferedOutputStream bufferedOut = new BufferedOutputStream(fileOutStream);

                        while ((len = bufferedIn.read(buffer, 0, 1024)) != -1) {
                            bufferedOut.write(buffer, 0, len);
                        }
                        bufferedOut.flush();
                        bufferedOut.close();
                    } catch (IOException ex) {
                        // In case of an IOException the connection will be released
                        // back to the connection manager automatically
                        throw ex;
                    } finally {
                        // Closing the input stream will trigger connection release
                        in.close();
                    }
                }
            } finally {
                response.close();
            }
        } finally {
            httpclient.close();
        }
    }

    public final static void main(String[] args) throws Exception {
        downloadFile("http://xtuer.github.io/img/dog.png", "/Users/Biao/Desktop/a.png"); // 下载图片
    }
}

参考

• HttpClient Tutorial
• HttpClient Examples