Java：使用HttpURLConnection发送请求时报文体为什么会为空？

最近使用java.net.HttpURLConnection做一些简单的测试程序，发现发送的报文体为空：

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;public class HttpURLConnectionTester {private HttpURLConnection connectServerConn() throws IOException {// set ServerServlet URLString urlPath = "http://127.0.0.1"+ ":7010" //+ "/wtcapp" //+ "/server";URL url = new URL(urlPath);HttpURLConnection conn = (HttpURLConnection) url.openConnection();System.out.println("HttpURLConnection: " + conn.getClass());conn.setDoOutput(true);conn.setRequestMethod("POST");conn.setRequestProperty("Content-Type", "text/plain");conn.connect();return conn;}public void doTest01() {try {HttpURLConnection conn = connectServerConn();OutputStream output = conn.getOutputStream();InputStream input = conn.getInputStream();     // (1)output.write("hello ".getBytes());output.flush();Thread.sleep(3000);output.write("world!".getBytes());output.flush();output.close();// (2)StringBuffer ret = new StringBuffer();int r = input.read();while (r != -1) {ret.append((char) r);r = input.read();}System.out.println("ret:" + ret);input.close();} catch (Exception e) {e.printStackTrace();}}public static void main(String[] args) {HttpURLConnectionTester tester = new HttpURLConnectionTester();tester.doTest01();}}

截取报文发现：

POST /wtcapp/server HTTP/1.1
Content-Type: text/plain
User-Agent: Java/1.8.0_261
Host: 127.0.0.1:7010
Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2
Connection: keep-alive
Content-Length: 0

Content-Length为0，报文体为空。

经过研究发现，只要把（1）行的代码，移动到（2）处理，问题就解决了：

  public void doTest01() {try {HttpURLConnection conn = connectServerConn();OutputStream output = conn.getOutputStream();output.write("hello ".getBytes());output.flush();Thread.sleep(3000);output.write("world!".getBytes());output.flush();output.close();InputStream input = conn.getInputStream();StringBuffer ret = new StringBuffer();int r = input.read();while (r != -1) {ret.append((char) r);r = input.read();}System.out.println("ret:" + ret);input.close();} catch (Exception e) {e.printStackTrace();}}

POST /wtcapp/server HTTP/1.1
Content-Type: text/plain
User-Agent: Java/1.8.0_261
Host: 127.0.0.1:7010
Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2
Connection: keep-alive
Content-Length: 12

hello world!

查看HttpURLConnection的源代码发现，原来conn.getInputStream();时，会将前面打开的OutputStream给close掉造成的。