增删改查写在一个servlet的例子

下面是一个将增删改查操作写在一个servlet中的例子：

import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class CRUDServlet extends HttpServlet {
    private List<String> data = new ArrayList<>();

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String action = req.getParameter("action");
        PrintWriter writer = resp.getWriter();

        if (action == null) {
            writer.println("Please specify an action.");
            return;
        }

        switch (action) {
            case "add":
                addData(req, writer);
                break;
            case "delete":
                deleteData(req, writer);
                break;
            case "update":
                updateData(req, writer);
                break;
            case "get":
                getData(writer);
                break;
            default:
                writer.println("Invalid action.");
        }
    }

    private void addData(HttpServletRequest req, PrintWriter writer) {
        String newData = req.getParameter("data");
        data.add(newData);
        writer.println("Data added successfully.");
    }

    private void deleteData(HttpServletRequest req, PrintWriter writer) {
        int index = Integer.parseInt(req.getParameter("index"));
        if (index >= 0 && index < data.size()) {
            data.remove(index);
            writer.println("Data deleted successfully.");
        } else {
            writer.println("Invalid index.");
        }
    }

    private void updateData(HttpServletRequest req, PrintWriter writer) {
        int index = Integer.parseInt(req.getParameter("index"));
        String newData = req.getParameter("data");
        if (index >= 0 && index < data.size()) {
            data.set(index, newData);
            writer.println("Data updated successfully.");
        } else {
            writer.println("Invalid index.");
        }
    }

    private void getData(PrintWriter writer) {
        writer.println("Data:");
        for (String entry : data) {
            writer.println(entry);
        }
    }
}

在这个例子中，我们使用doGet方法来处理GET请求，并根据action参数的值来执行相应的操作。addData方法用于添加数据，deleteData方法用于删除数据，updateData方法用于更新数据，getData方法用于获取数据。

你可以将上述代码编译为一个servlet，并部署到你的Web容器中。然后，通过发送GET请求到该servlet的URL，并传递相应的参数来执行增删改查操作。例如，你可以使用以下URL来添加数据：

http://localhost:8080/crud?action=add&data=example

你可以根据需要修改参数和URL来执行其他操作