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# Java怎么发起HTTP请求
HTTP请求是现代应用程序开发中最基础也最重要的功能之一。无论是调用第三方API、获取网络数据还是实现微服务间的通信，都需要使用HTTP协议。Java作为企业级开发的主流语言，提供了多种发起HTTP请求的方式。本文将全面介绍Java中发起HTTP请求的各类方法、最佳实践以及常见问题解决方案。
## 目录
1. [HTTP协议基础](#http协议基础)
2. [Java原生HTTP客户端](#java原生http客户端)
- [HttpURLConnection](#httpurlconnection)
- [HttpClient (Java 11+)](#httpclient-java-11)
3. [第三方HTTP客户端库](#第三方http客户端库)
- [Apache HttpClient](#apache-httpclient)
- [OkHttp](#okhttp)
- [RestTemplate](#resttemplate)
- [WebClient](#webclient)
4. [HTTP请求方法详解](#http请求方法详解)
- [GET请求](#get请求)
- [POST请求](#post请求)
- [PUT/DELETE请求](#putdelete请求)
5. [请求头与参数处理](#请求头与参数处理)
6. [响应处理](#响应处理)
7. [异步HTTP请求](#异步http请求)
8. [HTTPS与证书处理](#https与证书处理)
9. [性能优化](#性能优化)
10. [常见问题与解决方案](#常见问题与解决方案)
11. [总结与建议](#总结与建议)
## HTTP协议基础
HTTP(HyperText Transfer Protocol)是应用层协议，用于分布式、协作式和超媒体信息系统的应用层协议。HTTP是无状态协议，意味着服务器不会在两个请求之间保留任何数据(状态)。
主要特点：
- 基于请求/响应模型
- 简单快速
- 灵活(可以传输任意类型数据)
- 无连接(每次连接只处理一个请求)
- 无状态
HTTP请求由以下部分组成：
1. 请求行(方法、URL、协议版本)
2. 请求头(Header)
3. 空行
4. 请求体(Body)
## Java原生HTTP客户端
### HttpURLConnection
`HttpURLConnection`是Java最基础的HTTP客户端，存在于`java.net`包中，自JDK1.1就已存在。
```java
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
public class HttpURLConnectionExample {
public static void main(String[] args) throws Exception {
URL url = new URL("https://example.com/api");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
// 设置请求方法
connection.setRequestMethod("GET");
// 设置请求头
connection.setRequestProperty("User-Agent", "Java HTTP Client");
connection.setRequestProperty("Accept", "application/json");
// 获取响应码
int responseCode = connection.getResponseCode();
System.out.println("Response Code: " + responseCode);
// 读取响应内容
try (BufferedReader in = new BufferedReader(
new InputStreamReader(connection.getInputStream()))) {
String line;
StringBuilder response = new StringBuilder();
while ((line = in.readLine()) != null) {
response.append(line);
}
System.out.println("Response: " + response.toString());
}
// 关闭连接
connection.disconnect();
}
}
优点： - JDK内置，无需额外依赖 - 简单请求场景下使用方便
缺点： - API设计较为底层，使用繁琐 - 缺乏连接池管理 - 错误处理不够友好 - 不支持HTTP/2
Java 11引入了新的HttpClient，位于java.net.http包中，支持HTTP/2和WebSocket，提供了同步和异步两种调用方式。
import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
public class HttpClientExample {
public static void main(String[] args) throws Exception {
// 创建HttpClient实例
HttpClient client = HttpClient.newHttpClient();
// 构建请求
HttpRequest request = HttpRequest.newBuilder()
.uri(URI.create("https://example.com/api"))
.header("Content-Type", "application/json")
.GET()
.build();
// 发送同步请求
HttpResponse<String> response = client.send(
request, HttpResponse.BodyHandlers.ofString());
System.out.println("Status code: " + response.statusCode());
System.out.println("Response body: " + response.body());
// 异步请求示例
client.sendAsync(request, HttpResponse.BodyHandlers.ofString())
.thenApply(HttpResponse::body)
.thenAccept(System.out::println)
.join();
}
}
优点： - 现代API设计，使用流畅 - 支持HTTP/2 - 内置异步支持 - 更好的性能
缺点： - 需要Java 11+环境 - 某些高级功能仍需第三方库
Apache HttpClient是功能最全面的Java HTTP客户端之一，提供了丰富的功能和配置选项。
Maven依赖：
<dependency>
<groupId>org.apache.httpcomponents</groupId>
<artifactId>httpclient</artifactId>
<version>4.5.13</version>
</dependency>
使用示例：
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 org.apache.http.util.EntityUtils;
public class ApacheHttpClientExample {
public static void main(String[] args) throws Exception {
try (CloseableHttpClient httpClient = HttpClients.createDefault()) {
HttpGet request = new HttpGet("https://example.com/api");
// 设置请求头
request.addHeader("User-Agent", "Apache HttpClient");
try (CloseableHttpResponse response = httpClient.execute(request)) {
// 获取响应状态
System.out.println(response.getStatusLine().toString());
// 获取响应内容
HttpEntity entity = response.getEntity();
if (entity != null) {
String result = EntityUtils.toString(entity);
System.out.println(result);
}
}
}
}
}
特点： - 成熟的HTTP客户端实现 - 支持连接池管理 - 支持多种认证机制 - 支持HTTPS - 支持自定义拦截器
OkHttp是由Square公司开发的现代HTTP客户端，Android开发中广泛使用，但也适用于Java应用。
Maven依赖：
<dependency>
<groupId>com.squareup.okhttp3</groupId>
<artifactId>okhttp</artifactId>
<version>4.9.3</version>
</dependency>
使用示例：
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;
public class OkHttpExample {
public static void main(String[] args) throws Exception {
OkHttpClient client = new OkHttpClient();
Request request = new Request.Builder()
.url("https://example.com/api")
.addHeader("Accept", "application/json")
.build();
try (Response response = client.newCall(request).execute()) {
if (!response.isSuccessful()) {
throw new RuntimeException("Unexpected code " + response);
}
System.out.println(response.body().string());
}
}
}
特点： - 简洁的API设计 - 自动GZIP压缩 - 响应缓存 - 连接池复用 - 支持HTTP/2
RestTemplate是Spring框架提供的HTTP客户端，虽然已被标记为过时(推荐使用WebClient)，但在许多项目中仍有使用。
import org.springframework.web.client.RestTemplate;
public class RestTemplateExample {
public static void main(String[] args) {
RestTemplate restTemplate = new RestTemplate();
String url = "https://example.com/api";
// GET请求
String response = restTemplate.getForObject(url, String.class);
System.out.println(response);
// POST请求
// String requestBody = "{\"key\":\"value\"}";
// String postResponse = restTemplate.postForObject(url, requestBody, String.class);
}
}
特点： - 与Spring生态集成良好 - 自动对象映射(JSON/XML) - 支持OAuth等安全机制
WebClient是Spring 5引入的响应式HTTP客户端，支持异步和非阻塞IO。
Maven依赖：
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-webflux</artifactId>
</dependency>
使用示例：
import org.springframework.web.reactive.function.client.WebClient;
import reactor.core.publisher.Mono;
public class WebClientExample {
public static void main(String[] args) {
WebClient client = WebClient.create("https://example.com");
Mono<String> response = client.get()
.uri("/api")
.retrieve()
.bodyToMono(String.class);
// 阻塞获取结果(实际应用中通常不阻塞)
System.out.println(response.block());
}
}
特点： - 响应式编程模型 - 支持HTTP/2 - 非阻塞IO - 流式处理 - 与Spring生态深度集成
GET是最常见的HTTP方法，用于获取资源。
// 使用HttpURLConnection
URL url = new URL("https://example.com/api?param1=value1¶m2=value2");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("GET");
// 使用HttpClient
HttpRequest request = HttpRequest.newBuilder()
.uri(URI.create("https://example.com/api?param1=value1¶m2=value2"))
.GET()
.build();
POST用于提交数据到服务器。
// 使用HttpURLConnection
URL url = new URL("https://example.com/api");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("POST");
connection.setDoOutput(true);
String postData = "param1=value1¶m2=value2";
try (OutputStream os = connection.getOutputStream()) {
byte[] input = postData.getBytes("utf-8");
os.write(input, 0, input.length);
}
// 使用HttpClient
String requestBody = "{\"key\":\"value\"}";
HttpRequest request = HttpRequest.newBuilder()
.uri(URI.create("https://example.com/api"))
.header("Content-Type", "application/json")
.POST(HttpRequest.BodyPublishers.ofString(requestBody))
.build();
PUT用于更新资源，DELETE用于删除资源。
// PUT请求示例
HttpRequest putRequest = HttpRequest.newBuilder()
.uri(URI.create("https://example.com/api/123"))
.header("Content-Type", "application/json")
.PUT(HttpRequest.BodyPublishers.ofString("{\"name\":\"new value\"}"))
.build();
// DELETE请求示例
HttpRequest deleteRequest = HttpRequest.newBuilder()
.uri(URI.create("https://example.com/api/123"))
.DELETE()
.build();
// HttpURLConnection
connection.setRequestProperty("Content-Type", "application/json");
connection.setRequestProperty("Authorization", "Bearer token");
// HttpClient
HttpRequest request = HttpRequest.newBuilder()
.header("Content-Type", "application/json")
.header("Authorization", "Bearer token")
.build();
// 手动构建查询字符串
String query = "param1=" + URLEncoder.encode("value1", StandardCharsets.UTF_8) +
"¶m2=" + URLEncoder.encode("value2", StandardCharsets.UTF_8);
// 使用URIBuilder (Apache HttpClient)
URI uri = new URIBuilder("https://example.com/api")
.addParameter("param1", "value1")
.addParameter("param2", "value2")
.build();
// 使用URL编码表单
String form = "username=" + URLEncoder.encode("user", "UTF-8") +
"&password=" + URLEncoder.encode("pass", "UTF-8");
// 使用MultiValueMap (Spring)
MultiValueMap<String, String> formData = new LinkedMultiValueMap<>();
formData.add("username", "user");
formData.add("password", "pass");
// HttpURLConnection
int status = connection.getResponseCode();
if (status == HttpURLConnection.HTTP_OK) {
// 处理成功响应
} else {
// 处理错误
}
// HttpClient
HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
if (response.statusCode() == 200) {
System.out.println(response.body());
} else {
System.out.println("Error: " + response.statusCode());
}
// 使用Jackson
ObjectMapper mapper = new ObjectMapper();
MyObject obj = mapper.readValue(response.body(), MyObject.class);
// 使用Gson
Gson gson = new Gson();
MyObject obj = gson.fromJson(response.body(), MyObject.class);
// 获取字节数组
byte[] bytes = response.body().getBytes();
// 保存为文件
try (FileOutputStream fos = new FileOutputStream("file.png")) {
fos.write(bytes);
}
HttpClient client = HttpClient.newHttpClient();
HttpRequest request = HttpRequest.newBuilder()
.uri(URI.create("https://example.com/api"))
.build();
client.sendAsync(request, HttpResponse.BodyHandlers.ofString())
.thenApply(HttpResponse::body)
.thenAccept(System.out::println)
.exceptionally(e -> {
System.err.println("Error: " + e.getMessage());
return null;
});
// 主线程继续执行其他任务
System.out.println("Request sent asynchronously");
WebClient client = WebClient.create("https://example.com");
client.get()
.uri("/api")
.retrieve()
.bodyToMono(String.class)
.subscribe(
response -> System.out.println("Response: " + response),
error -> System.err.println("Error: " + error.getMessage())
);
// 创建信任所有证书的HttpClient
HttpClient client = HttpClient.newBuilder()
.sslContext(createUnsafeSSLContext())
.build();
private static SSLContext createUnsafeSSLContext() throws Exception {
TrustManager[] trustAllCerts = new TrustManager[]{
new X509TrustManager() {
public void checkClientTrusted(X509Certificate[] chain, String authType) {}
public void checkServerTrusted(X509Certificate[] chain, String authType) {}
public X509Certificate[] getAcceptedIssuers() { return new X509Certificate[0]; }
}
};
SSLContext sslContext = SSLContext.getInstance("TLS");
sslContext.init(null, trustAllCerts, new SecureRandom());
return sslContext;
}
// 加载自定义信任库
KeyStore keyStore = KeyStore.getInstance(KeyStore.getDefaultType());
try (InputStream is = new FileInputStream("truststore.jks")) {
keyStore.load(is, "password".toCharArray());
}
TrustManagerFactory tmf = TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm());
tmf.init(keyStore);
SSLContext sslContext = SSLContext.getInstance("TLS");
sslContext.init(null, tmf.getTrustManagers(), null);
HttpClient client = HttpClient.newBuilder()
.sslContext(sslContext)
.build();
// Apache HttpClient连接池配置
PoolingHttpClientConnectionManager connectionManager =
new PoolingHttpClientConnectionManager();
connectionManager.setMaxTotal(200); // 最大连接数
connectionManager.setDefaultMaxPerRoute(20); // 每个路由最大连接数
CloseableHttpClient httpClient = HttpClients.custom()
.setConnectionManager(connectionManager)
.build();
// HttpClient超时设置
HttpClient client = HttpClient.newBuilder()
.connectTimeout(Duration.ofSeconds(10))
.build();
// Apache HttpClient超时设置
RequestConfig config = RequestConfig.custom()
.setConnectTimeout(5000)
.setSocketTimeout(5000)
.build();
CloseableHttpClient httpClient = HttpClients.custom()
.setDefaultRequestConfig(config)
.build();
// OkHttp缓存配置
int cacheSize = 10 * 1024 * 1024; // 10MB
Cache cache = new Cache(new File("http-cache"), cacheSize);
OkHttpClient client = new OkHttpClient.Builder()
.cache(cache)
.build();
问题表现：java.net.ConnectException: Connection timed out
解决方案： - 检查网络连接 - 增加连接超时时间 - 检查目标服务器是否可达
// 设置连接超时
HttpClient client = HttpClient.newBuilder()
.connectTimeout(Duration.ofSeconds(30))
.build();
问题表现：javax.net.ssl.SSLHandshakeException
解决方案： - 导入正确的证书 - 开发环境可以临时禁用证书验证(不推荐生产环境使用)
问题表现：com.fasterxml.jackson.core.JsonParseException
解决方案： - 检查响应内容是否符合预期格式 - 添加错误处理逻辑
try {
ObjectMapper mapper = new ObjectMapper();
MyObject obj = mapper.readValue(response, MyObject.class);
} catch (JsonProcessingException e) {
System.err.println("Failed to parse JSON: " + e.getMessage());
// 处理错误情况
}
问题表现：应用逐渐变慢，最终耗尽连接资源
解决方案： - 确保所有资源都被正确关闭 -
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