Java GZip压缩详解

一、GZip压缩介绍

GZip是一种数据压缩算法，它使用Lempel-Ziv算法(LZ77)和哈夫曼编码来压缩数据。它可以压缩任何数据类型，但最适用于纯文本文件，例如HTML、CSS和JavaScript文件。GZip使用DEFLATE算法来实现数据压缩和解压缩。

在Java中，GZip压缩功能由java.util.zip.GZIPOutputStream和java.util.zip.GZIPInputStream类提供。

二、GZip压缩的优点

1、文件大小明显缩小。对于大型文本文件（如日志文件、XML文件等），GZip压缩可以将文件大小缩小到原来的1/3或1/4。

2、网络传输效率提高。通过使用GZip压缩，可以减少数据传输量，从而提高网络传输效率，特别是在低速网络或网络拥塞情况下。

3、更安全的数据传输。GZip压缩可以提高数据的安全性，因为压缩后的文件更难以解密。

三、使用GZip压缩和解压缩文件

GZip压缩和解压缩文件的方法如下：

import java.util.zip.*;
import java.io.*;

public class GZipDemo {

    public static void main(String[] args) {
        String fileToCompress = "example.txt";
        String compressedFile = "example.txt.gz";
        String decompressedFile = "exampleCopy.txt";

        compressFile(fileToCompress, compressedFile);
        decompressFile(compressedFile, decompressedFile);
    }

    public static void compressFile(String fileToCompress, String compressedFile) {
        try {
            FileInputStream fileInputStream = new FileInputStream(fileToCompress);
            FileOutputStream fileOutputStream = new FileOutputStream(compressedFile);
            GZIPOutputStream gzipOutputStream = new GZIPOutputStream(fileOutputStream);

            byte[] buffer = new byte[1024];
            int bytesRead;

            while ((bytesRead = fileInputStream.read(buffer)) > 0) {
                gzipOutputStream.write(buffer, 0, bytesRead);
            }

            fileInputStream.close();

            gzipOutputStream.finish();
            gzipOutputStream.close();

            System.out.println("File " + fileToCompress + " has been compressed to " + compressedFile + ".");
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }

    public static void decompressFile(String compressedFile, String decompressedFile) {
        try {
            FileInputStream fileInputStream = new FileInputStream(compressedFile);
            GZIPInputStream gzipInputStream = new GZIPInputStream(fileInputStream);
            FileOutputStream fileOutputStream = new FileOutputStream(decompressedFile);

            byte[] buffer = new byte[1024];
            int bytesRead;

            while ((bytesRead = gzipInputStream.read(buffer)) > 0) {
                fileOutputStream.write(buffer, 0, bytesRead);
            }

            gzipInputStream.close();
            fileOutputStream.close();

            System.out.println("File " + compressedFile + " has been decompressed to " + decompressedFile + ".");
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }
}

上述示例代码演示了如何压缩和解压缩文件。首先，我们创建了一个包含要压缩的文件的字符串，一个包含压缩文件的字符串和一个包含解压缩文件的字符串。然后，我们使用compressFile()方法压缩原始文件，并使用decompressFile()方法解压缩压缩文件。

四、GZip压缩HTTP响应

GZip压缩可以通过HTTP传输减少响应大小，从而提高网络传输效率。可以使用Java Servlet API的GZipFilter类来轻松实现GZip压缩HTTP响应。

以下是一个使用GZipFilter类进行HTTP响应压缩的示例：

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletResponseWrapper;
import javax.servlet.http.HttpServlet;

public class GZipServlet extends HttpServlet {

    static final String ENCODING_HEADER = "Content-Encoding";
    static final String ENCODING = "gzip";

    public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) res;

        String acceptEncoding = request.getHeader("Accept-Encoding");
        if (acceptEncoding != null && acceptEncoding.indexOf(ENCODING) >= 0) {
            GZipServletResponseWrapper gzipResponse = new GZipServletResponseWrapper(response);
            gzipResponse.setCompressionThreshold(256);
            try {
                super.service(request, gzipResponse);
            } finally {
                gzipResponse.finishResponse();
            }
            return;
        }

        super.service(request, response);
    }

    private static class GZipServletResponseWrapper extends HttpServletResponseWrapper {

        protected HttpServletResponse response = null;
        protected GZipServletOutputStream gzipOutputStream = null;
        protected PrintWriter printWriter = null;
        protected int compressionThreshold = 0;

        public GZipServletResponseWrapper(HttpServletResponse response) throws IOException {
            super(response);
            this.response = response;
        }

        public void setCompressionThreshold(int compressionThreshold) {
            this.compressionThreshold = compressionThreshold;
        }

        private void finishResponse() throws IOException {
            if (this.printWriter != null) {
                this.printWriter.close();
            }
            if (this.gzipOutputStream != null) {
                this.gzipOutputStream.close();
            }
        }

        public void flushBuffer() throws IOException {
            if (this.printWriter != null) {
                this.printWriter.flush();
            }
            IOException exception1 = null;
            try {
                if (this.gzipOutputStream != null) {
                    this.gzipOutputStream.flush();
                }
            } catch (IOException ex) {
                exception1 = ex;
            }
            IOException exception2 = null;
            try {
                super.flushBuffer();
            } catch (IOException ex) {
                exception2 = ex;
            }
            if (exception1 != null) {
                throw exception1;
            }
            if (exception2 != null) {
                throw exception2;
            }
        }

        public ServletOutputStream getOutputStream() throws IOException {
            if (this.printWriter != null) {
                throw new IllegalStateException("PrintWriter obtained already - cannot get OutputStream");
            }
            if (this.gzipOutputStream == null) {
                this.gzipOutputStream = new GZipServletOutputStream(this.response.getOutputStream());
                this.response.addHeader(ENCODING_HEADER, ENCODING);
            }
            return this.gzipOutputStream;
        }

        public PrintWriter getWriter() throws IOException {
            if (this.printWriter == null && this.gzipOutputStream != null) {
                throw new IllegalStateException("OutputStream obtained already - cannot get PrintWriter");
            }
            if (this.printWriter == null) {
                this.gzipOutputStream = new GZipServletOutputStream(this.response.getOutputStream());
                this.response.addHeader(ENCODING_HEADER, ENCODING);
                this.printWriter = new PrintWriter(new OutputStreamWriter(this.gzipOutputStream, this.response.getCharacterEncoding()));
            }
            return this.printWriter;
        }
    }

    private static class GZipServletOutputStream extends ServletOutputStream {

        protected OutputStream outputStream = null;

        public GZipServletOutputStream(OutputStream outputStream) throws IOException {
            super();
            this.outputStream = outputStream;
        }

        public void close() throws IOException {
            this.outputStream.close();
            super.close();
        }

        public void flush() throws IOException {
            this.outputStream.flush();
        }

        public void write(byte[] b) throws IOException {
            this.outputStream.write(b);
        }

        public void write(byte[] b, int off, int len) throws IOException {
            this.outputStream.write(b, off, len);
        }

        public void write(int b) throws IOException {
            this.outputStream.write(b);
        }
    }
}

上述示例代码演示了如何使用GZipFilter类将HTTP响应压缩并发送给客户端。在GZipServlet中，我们首先检查浏览器是否支持GZip压缩，如果支持，我们将响应包装在一个GZipServletResponseWrapper中，然后使用GZipServletOutputStream类将响应压缩。最后，我们在响应头中添加Content-Encoding：gzip标头，并发送响应。