---
title: "循序渐进写一个Servlet(3) - 分别处理GET和POST"
source: https://www.boris1993.com/java-servlet-3-handle-get-and-post.html
date: 2019-03-08
updated: 2026-10-10
tags: [Java, Servlet]
categories: [学知识]
---

# 循序渐进写一个Servlet(3) - 分别处理GET和POST

Servlet（Server Applet），全称Java Servlet，是用Java编写的服务器端程序。其主要功能在于交互式地浏览和修改数据，生成动态Web内容。本系列将一步步地写出一个Servlet程序。

这篇博文将演示如何分别处理`GET`和`POST`请求，以及处理请求中的参数。

# 编写`doGet()`和`doPost()`方法

首先把要实现的功能写好，后面才好调用不是。

```java
public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
    // 设定返回内容的MIME类型
    response.setContentType("text/html");

    // 设定内容以UTF-8编码
    response.setCharacterEncoding("utf-8");

    try (PrintWriter writer = response.getWriter()) {
        // 开始输出HTML文本
        writer.print("<html lang=\"en\">");
        writer.print("<body>");
        writer.print("<b>Response from DemoServlet</b>");
        writer.print("<b>Handled by <code>doGet()</code></b>");
        writer.print("</body>");
        writer.print("</html>");
    }
}
```

```java
public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException {
    // 设定返回内容的MIME类型
    response.setContentType("text/html");

    // 设定内容以UTF-8编码
    response.setCharacterEncoding("utf-8");

    try (PrintWriter writer = response.getWriter()) {
        // 开始输出HTML文本
        writer.print("<html lang=\"en\">");
        writer.print("<body>");
        writer.print("<b>Response from DemoServlet</b>");
        writer.print("<b>Handled by <code>doPost()</code></b>");
        writer.print("</body>");
        writer.print("</html>");
    }
}
```

# 区分HTTP方法

因为servlet是调用`service()`方法来处理请求的，所以对请求做区分也需要在`service()`方法中进行。

```java
@Override
public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {

    HttpServletRequest httpServletRequest = (HttpServletRequest) req;
    HttpServletResponse httpServletResponse = (HttpServletResponse) res;

    if ("GET".equalsIgnoreCase(httpServletRequest.getMethod())) {
        doGet(httpServletRequest, httpServletResponse);
    } else if ("POST".equalsIgnoreCase(httpServletRequest.getMethod())) {
        doPost(httpServletRequest, httpServletResponse);
    } else {
        // 如果请求既不是GET也不是POST
        // 那么就返回HTTP 501 NOT IMPLEMENTED状态码
        // 毕竟不能把请求直接扔了，总是要有个返回的
        httpServletResponse.sendError(501);
    }
}
```

# 运行起来看看效果

首先发个`GET`请求

![Handling GET request](https://blog-static.boris1993.com/java-servlet-3-handle-get-and-post/handling-get-request.png)

再发个`POST`请求

![Handling POST request](https://blog-static.boris1993.com/java-servlet-3-handle-get-and-post/handling-post-request.png)

# 为什么不用`HttpServlet`类呢

没错，上面做的，就是自己实现了一个简陋的`HttpServlet`类，因为是循序渐进嘛，没头没脑的直接砸上来一个，算什么循序渐进。

那么现在就让`DemoServlet`继承`HttpServlet`。同时，因为`HttpServlet`已经在`service()`方法中实现了判断请求类型，所以`DemoServlet`中不要覆盖`service()`方法，只覆盖`doGet()`和`doPost()`方法。

```java
public class DemoServlet extends HttpServlet {

    @Override
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
        // 设定返回内容的MIME类型
        response.setContentType("text/html");

        // 设定内容以UTF-8编码
        response.setCharacterEncoding("utf-8");

        try (PrintWriter writer = response.getWriter()) {
            // 开始输出HTML文本
            writer.print("<html lang=\"en\">");
            writer.print("<body>");
            writer.print("<b>Response from DemoServlet</b>");
            writer.print("<b>Handled by <code>doGet()</code></b>");
            writer.print("</body>");
            writer.print("</html>");
        }
    }

    @Override
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException {
        // 设定返回内容的MIME类型
        response.setContentType("text/html");

        // 设定内容以UTF-8编码
        response.setCharacterEncoding("utf-8");

        try (PrintWriter writer = response.getWriter()) {
            // 开始输出HTML文本
            writer.print("<html lang=\"en\">");
            writer.print("<body>");
            writer.print("<b>Response from DemoServlet</b>");
            writer.print("<b>Handled by <code>doPost()</code></b>");
            writer.print("</body>");
            writer.print("</html>");
        }
    }
}
```

# 处理请求中的参数

HTTP请求是可以带参数的，有了参数，那就得处理。

## 处理`GET`请求的参数

`GET`请求里带的参数，名字叫`查询字符串(query string)`，是一组或多组`key=value`格式的键值对。

Query string写在URL后面，以一个问号起头，用`&`分隔各个键值对，即类似`http://localhost:8080/appname/servlet?arg1=value1&arg2=value2&...&argN=valueN`。

在代码里使用`HttpServletRequest#getQueryString()`方法，就可以获取到问号后面的query string，分别用`&`和`=`分割字符串，就可以取到每个参数的key和value。

```java
@Override
public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
    // 设定返回内容的MIME类型
    response.setContentType("text/html");

    // 设定内容以UTF-8编码
    response.setCharacterEncoding("utf-8");

    /*

    String queryString = request.getQueryString();
    String[] queryStrings;

    if (queryString != null) {
        queryStrings = queryString.split("&");
    } else {
        queryStrings = new String[]{};
    }

    */

    // 使用Optional类简化null判断
    Optional<String> optionalQueryString = Optional.ofNullable(request.getQueryString());

    // 如果有query string则取出每个参数
    // 如果没有则返回一个空的String数组
    String[] queryStrings = optionalQueryString.isPresent() ? optionalQueryString.get().split("&") : new String[]{};

    try (PrintWriter writer = response.getWriter()) {
        // 开始输出HTML文本
        writer.print("<html lang=\"en\">");
        writer.print("<body>");
        writer.print("<b>Response from DemoServlet</b>");
        writer.print("<b>Handled by <code>doGet()</code></b>");

        writer.print("<br>");

        // 遍历每个参数
        for (String query : queryStrings) {
            // 取出参数的key和value
            String[] q = query.split("=");

            writer.print(q[0] + " = " + q[1] + "<br>");
        }

        writer.print("<br>");

        writer.print("</body>");
        writer.print("</html>");
    }
}
```

运行一下，结果是这样子的：

![Handling query string](https://blog-static.boris1993.com/java-servlet-3-handle-get-and-post/handling-query-string.png)

## 处理`POST`请求的参数

`POST`请求的参数就叫参数(parameter)，位于请求体(body)里，格式由`Content-Type`请求头决定。详细介绍可以参考这篇MDN文档。

`HttpServletRequest#getParameterMap()`方法可以取出请求中的所有参数，并放到一个Map中。

```java
@Override
public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException {
    // 设定返回内容的MIME类型
    response.setContentType("text/html");

    // 设定内容以UTF-8编码
    response.setCharacterEncoding("utf-8");

    // 取出所有参数，得到一个Map
    Map parameterMap = request.getParameterMap();

    try (PrintWriter writer = response.getWriter()) {
        // 开始输出HTML文本
        writer.print("<html lang=\"en\">");
        writer.print("<body>");
        writer.print("<b>Response from DemoServlet</b>");
        writer.print("<b>Handled by <code>doPost()</code></b>");

        writer.print("<br>");

        // 遍历parameterMap
        parameterMap.forEach((k, v) -> writer.print(k + " = " + ((String[]) v)[0] + "<br>"));

        writer.print("</body>");
        writer.print("</html>");
    }
}
```

本文中将使用`application/x-www-form-urlencoded`格式做示例。

![Handling parameter](https://blog-static.boris1993.com/java-servlet-3-handle-get-and-post/handling-parameter.png)

# 系列博文

-   [循序渐进写一个Servlet(1) - 介绍相关的接口和类](https://www.boris1993.com/projects/java/Servlet/java-servlet-1-introducing-classes-and-interfaces.html)
-   [循序渐进写一个Servlet(2) - 第一个servlet](https://www.boris1993.com/projects/java/Servlet/java-servlet-2-first-servlet.html)
-   [循序渐进写一个Servlet(3) - 分别处理GET和POST](https://www.boris1993.com/projects/java/Servlet/java-servlet-3-handle-get-and-post.html)
-   [循序渐进写一个Servlet(4) - 会话追踪](https://www.boris1993.com/projects/java/Servlet/java-servlet-4-session-tracking.html)
-   [循序渐进写一个Servlet(5) - Filter](https://www.boris1993.com/projects/java/Servlet/java-servlet-5-filter.html)