纯Servlet上传文件

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class UploadServlet extends HttpServlet {

    private static final long serialVersionUID = -6743769305857881099L;
    // default maximum allowable file size is 1000k

    static final int MAX_SIZE = 1024000;
    // instance variables to store root and success message

    String rootPath, successMessage;
    FileWriter savefile;
    String filename = null;
    String value = null;

    /**
     * init method is called when servlet is initialized.
     */
    public void init(ServletConfig config) throws ServletException {
        super.init(config);
        // get path in which to save file

        rootPath = config.getInitParameter("RootPath");
        if (rootPath == null) {
            rootPath = "/";
        }
        /*
         * Get message to show when upload is complete. Used only if a success redirect page is not supplied.
         */
        successMessage = config.getInitParameter("SuccessMessage");
        if (successMessage == null) {
            successMessage = "File upload complete!";
        }
    }

    /**
     * doPost reads the uploaded data from the request and writes it to a file.
     */
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        response.setContentType("text/html");
        response.setCharacterEncoding("GB2312");
        PrintWriter out = response.getWriter();
        // FileWriter savefile;

        try {
            // Verify the content type

            String ct = request.getContentType();
            if (!ct.startsWith("multipart/form-data"))
                throw new RuntimeException("Invalid content type");
            // Get the boundary string

            int p = ct.indexOf("boundary=");
            if (p == -1)
                throw new RuntimeException("No boundary string found");
            p += "boundary=".length();
            String boundary = "--" + ct.substring(p);
            String finalBoundary = boundary + "--";
            // We 'll parse the multipart/form-data

            // with a finite state machine

            // Define names for the parser states

            final int INIT = 0;
            final int READING_HEADERS = 1;
            final int READING_DATA = 2;
            int state = INIT;
            // Read and extract the fields

            BufferedReader in = request.getReader();
            main: for (;;) {
                String line = in.readLine();
                if (line == null)
                    break;
                switch (state) {
                // State 0: Ignoring everything before

                // the first boundary

                case INIT:
                    if (line.startsWith(finalBoundary))
                        break main;
                    if (line.startsWith(boundary)) {
                        state = READING_HEADERS;
                        filename = "";
                        value = "";
                    }
                    break;
                // State 1: Parsing the headers

                case READING_HEADERS:
                    if (line.length() == 0)
                        state = READING_DATA;
                    else {
                        // Get the field name

                        p = line.indexOf("filename=\"");
                        if (p == -1)
                            break;
                        p += "filename=\"".length();

                        // ... up to the closing quote.


                        int q = line.indexOf("\"", p);
                        if (q == -1)
                            break;
                        filename = line.substring(p, q);
                        filename = request.getRealPath("/") + filename.substring(filename.lastIndexOf("\\") + 1);
                        savefile = new FileWriter(filename);
                        value = "";
                    }
                    break;

                // State 2: Reading the data


                case READING_DATA:
                    if (line.startsWith(finalBoundary)) {
                        savefile.write(value);
                        savefile.close();
                        break main;
                    }
                    if (line.startsWith(boundary)) {
                        state = READING_HEADERS;
                    } else {
                        if (value.length() > 0)
                            value += "\n";
                        value += line;
                    }
                    break;
                }

            }
            // Report the incident number back to the client

            String[] text = { "", "",
                    "", "文件上传成功 ",
                    "", "", "", "文件上传成功!", "" };
            for (int i = 0; i < text.length; i++) {
                out.println(text[i]);
            }
            out.println(filename);
            out.println("
");
            out.println("");
        }

        catch (Exception e) {
            // Write the exception message

            out.println(" Error: ");
            out.println("");
            out.println(e.getMessage());
            out.println("");
        } finally {
            out.close();
        }
    }
}

纯Servlet上传文件，不用Apache的FileUpload。不过使用的时候建议使用FileUpload组件，有现成的为什么不用呢？