通过向服务器端发送get请求来让服务器端将其上的文件下载到客户端

紧接着上两篇关于文件流传递的demo，后来发现还是通过http的get请求，效果最好，可以很快的将服务器端的文件下载到客户端。代码如下所示：

首先我们写一个服务器端应用，它可以看成是一个servlet：

package download;
import java.io.BufferedInputStream;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
 * Servlet implementation class A
 */
public class ServerServlet extends HttpServlet {
	public static int BUFFER_SIZE = 1024 * 1024;
	static Runtime get_runtime = Runtime.getRuntime();
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse response) throws ServletException, IOException {
		usedMemory(1);
		File file = new File("E:/toodou_work/test.csv");
		BufferedInputStream in = new BufferedInputStream(new FileInputStream(file));
		//实现了一个输出流，其中的数据被写入一个 byte 数组。缓冲区会随着数据的不断写入而自动增长。可使用 toByteArray() 和 toString() 获取数据。 
		ByteArrayOutputStream out = new ByteArrayOutputStream(BUFFER_SIZE);
		byte[] temp = new byte[BUFFER_SIZE];
		int size = 0;
		//读入缓冲区的字节总数，如果因为已经到达文件末尾而没有更多的数据，则返回 -1。
		while ((size = in.read(temp)) != -1) {
			out.write(temp, 0, size);
		}
		usedMemory(2);
		in.close();
		//Provides an output stream for sending binary data to the client.
		ServletOutputStream baos = response.getOutputStream();
		out.writeTo(baos);// byte 数组输出流的全部内容写入到指定的输出流参数中，这与使用 out.write(buf, 0, count) 调用该输出流的 write 方法效果一样。 
		out.flush();
	}
	public static void usedMemory(int i) {
		long l = get_runtime.totalMemory() - get_runtime.freeMemory();
		long k = l / 1024;
		System.out.println(i + ": " + k + " K");
	}
	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		doGet(req, resp);
	}
}

客户端代码：

package download;
import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
public class SendGetRequest {
	static String sessionId = "";
	static Runtime send_runtime = Runtime.getRuntime();
	public static void main(String[] args) throws Exception {
		ServerServlet.usedMemory(3);
		URL url = new URL("http://localhost:8080/abc/ServerServlet");
		HttpURLConnection connection = (HttpURLConnection) url.openConnection();
		//URL 连接可用于输入和/或输出。如果打算使用 URL 连接进行输入，则将 DoInput 标志设置为 true；如果不打算使用，则设置为 false。默认值为 true。
		connection.setDoOutput(true);
		// Read from the connection. Default is true.
		connection.setDoInput(true);
		// Set the post method. Default is GET
		connection.setRequestMethod("GET");
		// URLConnection.setInstanceFollowRedirects是成员函数，仅作用于当前函数
		connection.setInstanceFollowRedirects(false);
		// connection. Settings above must be set before connect!
		// 配置本次连接的Content-type，配置为application/x-www-form-urlencoded的
		// 意思是正文是urlencoded编码过的form参数，下面我们可以看到我们对正文内容使用URLEncoder.encode
		// 进行编码
		connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
		connection.setRequestProperty("ENCTYPE", "multipart/form-data");
		// 连接，从postUrl.openConnection()至此的配置必须要在connect之前完成，
		// 要注意的是connection.getOutputStream会隐含的进行connect。
		connection.connect();
		/*//记录发送请求时间
		Long sendTime = System.currentTimeMillis();
		DataOutputStream out = new DataOutputStream(connection.getOutputStream());
		// 要传的参数
		String content = URLEncoder.encode("sendTime", "UTF-8") + "=" + URLEncoder.encode(sendTime.toString(), "UTF-8");
		//读海量文件
		StringBuilder postStrBuf = new StringBuilder();
		for (int i = 0; i < 1; i++) {
			File file = new File("E:/toodou_work/yuliang.csv");
			BufferedReader reader = null;
			String postStr = null;
			reader = new BufferedReader(new FileReader(file));
			while ((postStr = reader.readLine()) != null) {
				postStrBuf.append(postStr);
			}
		}
		content = content + "&" + URLEncoder.encode("file", "UTF-8") + "=" + URLEncoder.encode(postStrBuf.toString(), "UTF-8");
		// DataOutputStream.writeBytes将字符串中的16位的unicode字符以8位的字符形式写道流里面
		out.writeBytes(content);
		out.flush();
		out.close(); // flush and close
		*/
		//记录发送请求时间
		long start = System.currentTimeMillis();
		//Get Session ID
		String key = "";
		if (connection != null) {
			for (int i = 1; (key = connection.getHeaderFieldKey(i)) != null; i++) {
				if (key.equalsIgnoreCase("set-cookie")) {
					sessionId = connection.getHeaderField(key);
					sessionId = sessionId.substring(0, sessionId.indexOf(";"));
				}
			}
		}
		InputStream is = connection.getInputStream();
		BufferedInputStream bis = new BufferedInputStream(is);
		FileOutputStream fos = new FileOutputStream(new File("e:/1111111.txt"));
		BufferedOutputStream bos = new BufferedOutputStream(fos);
		byte[] temp = new byte[ServerServlet.BUFFER_SIZE];
		int size = 0;
		while ((size = bis.read(temp)) != -1) {
			bos.write(temp, 0, size);
		}
		ServerServlet.usedMemory(4);
		bos.flush();//刷新此缓冲的输出流。这迫使所有缓冲的输出字节被写出到底层输出流中。
		bos.close();
		bis.close();
		//		BufferedReader br = new BufferedReader(new InputStreamReader(is));
		//
		//		String str = null;
		//		StringBuilder sb = new StringBuilder();
		//
		//		while ((str = br.readLine()) != null) {
		//			System.out.println(str);
		//		}
		//记录将从连接请求返回的数据写到文件完毕的时间
		/*long end = System.currentTimeMillis();
		System.out.println("cost time: " + (end - start) + " ms");*/
		//关闭连接
		connection.disconnect();
	}
}