Java中遇HTTP 400错误时如何读取响应体？

我用Java代码调用HTTP POST接口时，响应码200能正常获取InputStream读取响应，但响应码400时，在标记/// error !!!的代码行会抛出异常：

Exception in thread "main" java.io.IOException: Server returned HTTP response code: 400 for URL: ...
at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1924)
at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1520)
at com.test123.Main009.main(Main009.java:41)

请问有没有办法读取4xx状态码对应的响应体？

对应的原代码：

package com.test123;

import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;
import java.nio.charset.StandardCharsets;
import java.util.Base64;

public class Main009 {

    public static void main(String[] args) throws Exception {
        String username = "xxx";
        String password = "yyy";
        
        URL url = new URL("http://host/path");
        URLConnection uc = url.openConnection();
        HttpURLConnection http = (HttpURLConnection)uc;
        
        http.setRequestMethod("POST"); // PUT is another valid option
        http.setDoOutput(true);
        http.setDoInput(true);

        byte[] request = "{ id: \"test2\" }".getBytes(StandardCharsets.UTF_8);
        int length = request.length;

        http.setFixedLengthStreamingMode(length);
        
        String userpass = username + ":" + password;
        String basicAuth = "Basic " + new String(Base64.getEncoder().encode(userpass.getBytes()));
        
        http.setRequestProperty ("Authorization", basicAuth);
        http.setRequestProperty("Content-Type", "application/json; charset=UTF-8");
        // http.connect();
        
        try(OutputStream os = http.getOutputStream()) {
            os.write(request);
            os.flush();
            
            InputStream in = http.getInputStream(); /// error !!!  
            System.out.println(in);
        }
    }
}

当HTTP响应码为4xx或5xx时，HttpURLConnection.getInputStream()会直接抛出异常，此时需要调用HttpURLConnection.getErrorStream()来读取错误响应体。核心处理逻辑如下：

• 先获取响应码，根据响应码区间判断使用正常输入流还是错误流
• 统一读取流内容，完成资源释放

修改后的代码示例：

package com.test123;

import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.nio.charset.StandardCharsets;
import java.util.Base64;

public class Main009 {

    public static void main(String[] args) throws Exception {
        String username = "xxx";
        String password = "yyy";
        
        URL url = new URL("http://host/path");
        HttpURLConnection http = (HttpURLConnection) url.openConnection();
        
        http.setRequestMethod("POST");
        http.setDoOutput(true);
        http.setDoInput(true);

        byte[] request = "{ id: \"test2\" }".getBytes(StandardCharsets.UTF_8);
        int length = request.length;

        http.setFixedLengthStreamingMode(length);
        
        String userpass = username + ":" + password;
        String basicAuth = "Basic " + new String(Base64.getEncoder().encode(userpass.getBytes()));
        
        http.setRequestProperty("Authorization", basicAuth);
        http.setRequestProperty("Content-Type", "application/json; charset=UTF-8");
        
        // 发送请求
        try(OutputStream os = http.getOutputStream()) {
            os.write(request);
            os.flush();
        }

        // 根据响应码选择对应输入流
        int responseCode = http.getResponseCode();
        InputStream inputStream = (responseCode >= 200 && responseCode < 300) 
            ? http.getInputStream() 
            : http.getErrorStream();

        // 读取并打印响应内容
        try(BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, StandardCharsets.UTF_8))) {
            StringBuilder response = new StringBuilder();
            String line;
            while ((line = reader.readLine()) != null) {
                response.append(line);
            }
            System.out.println("响应码: " + responseCode);
            System.out.println("响应内容: " + response.toString());
        } finally {
            http.disconnect();
        }
    }
}

1. 流的选择时机：必须先完成请求发送（写入输出流并关闭/flush），再获取响应码和对应流，否则会出现连接状态异常。
2. 错误流的有效性：getErrorStream()仅在响应码为错误状态（4xx/5xx）时返回有效流，成功状态下调用会返回null。
3. 资源释放：务必通过try-with-resources或手动关闭流，最后调用disconnect()释放HTTP连接资源。

内容的提问来源于stack exchange，提问作者peter.petrov