GZip Servlet过滤器可用于GZip压缩内容从Java Web应用程序发送到浏览器。
为什么要压缩
Gzip压缩HTML、js、css等，使得发送给浏览器的数据大小变得更小。提升上传速度，尤其是移动端带宽受限制的情况下，不过它可能带来服务器和浏览器的CPU消耗问题，但是响应速度会得道很大的改善。
GZip 请求头
浏览器在发送到HTTP服务器（例如Java Web服务器）的请求中包含Accept-Encoding HTTP标头。 Accept-Encoding标头的内容告诉浏览器可以接受哪些内容编码。 如果该标题包含gzip值，则浏览器可以接受GZip压缩内容。 然后服务器可以将GZip压缩发送回浏览器的内容。
如果从服务器发回的内容是GZip压缩的，则服务器会在HTTP响应中包含带有值gzip的Content-Encoding HTTP标头。 这样浏览器就知道内容是GZip压缩的。
为什么使用GZip Servlet过滤器？
如果对每一个Servlet请求都设置压缩，那肯定在性能上会有差距，所以Gzip Servlet过滤器 可以让我们对需要压缩的东西进行压缩，没必要压缩的就不去压缩。使性能最大化的提升。
GZip Servlet滤波器设计
image.png
如图所示：首先需要一个Servlet过滤器类。 该类映射到web.xml文件中的一组URL。
当一个HTTP请求到达映射到过滤器的Servlet容器时，过滤器会在该请求被Servlet，JSP等处理之前截取请求所针对的目标。 GZip servlet过滤器检查客户端（浏览器）是否可以接受GZip压缩内容。如果可以接受，就对目标做压缩处理，然后将HttpServletResponse对象封装在GZipServletResponseWrapper进行压缩，最后将压缩内容写入HttpServletResponse。
实例
The code consists of 3 classes. A GZipServletFilter, a GZipServletResponseWrapper and a GZipServletOutputStream.
The GZipServletOutputStream is what compresses the content written to it. It does so by using a GZIPOutputStream internally, which is a standard Java class.
主要用到三个类：GzipServletFilter、GzipServletResponseWrapper、GzipServletOutputStream。
- GZipServletFilter 用来拦截请求，检查浏览器是否接受压缩。它将在HttpServletResponse传递给过滤器链之前将信息包装在GZipServletResponseWrapper中。
- GZipServletResponseWrapper 用来返回一个输出流给Servlet或者Jsp，可以是GZipServletOutputStream 或者PrintWriter 类型的数据。
- GZipServletOutputStream 是用来压缩并写入的内容的，通过内部使用GZIPOutputStream来实现。
示例：
public class GZipServletFilter implements Filter {
@Override
public void init(FilterConfig filterConfig) throws ServletException {
}
@Override
public void destroy() {
}
public void doFilter(ServletRequest request,
ServletResponse response,
FilterChain chain)
throws IOException, ServletException {
HttpServletRequest httpRequest = (HttpServletRequest) request;
HttpServletResponse httpResponse = (HttpServletResponse) response;
if ( acceptsGZipEncoding(httpRequest) ) {
httpResponse.addHeader("Content-Encoding", "gzip");
GZipServletResponseWrapper gzipResponse =
new GZipServletResponseWrapper(httpResponse);
chain.doFilter(request, gzipResponse);
gzipResponse.close();
} else {
chain.doFilter(request, response);
}
}
//判断 请求对象 是否接受压缩
private boolean acceptsGZipEncoding(HttpServletRequest httpRequest) {
String acceptEncoding =
httpRequest.getHeader("Accept-Encoding");
return acceptEncoding != null &&
acceptEncoding.indexOf("gzip") != -1;
}
}
class GZipServletResponseWrapper extends HttpServletResponseWrapper {
private GZipServletOutputStream gzipOutputStream = null;
private PrintWriter printWriter = null;
public GZipServletResponseWrapper(HttpServletResponse response)
throws IOException {
super(response);
}
public void close() throws IOException {
//PrintWriter.close does not throw exceptions.
//Hence no try-catch block.
if (this.printWriter != null) {
this.printWriter.close();
}
if (this.gzipOutputStream != null) {
this.gzipOutputStream.close();
}
}
/**
* Flush OutputStream or PrintWriter
*
* @throws IOException
*/
@Override
public void flushBuffer() throws IOException {
//PrintWriter.flush() does not throw exception
if(this.printWriter != null) {
this.printWriter.flush();
}
IOException exception1 = null;
try{
if(this.gzipOutputStream != null) {
this.gzipOutputStream.flush();
}
} catch(IOException e) {
exception1 = e;
}
IOException exception2 = null;
try {
super.flushBuffer();
} catch(IOException e){
exception2 = e;
}
if(exception1 != null) throw exception1;
if(exception2 != null) throw exception2;
}
@Override
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(
getResponse().getOutputStream());
}
return this.gzipOutputStream;
}
@Override
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(
getResponse().getOutputStream());
this.printWriter = new PrintWriter(new OutputStreamWriter(
this.gzipOutputStream, getResponse().getCharacterEncoding()));
}
return this.printWriter;
}
@Override
public void setContentLength(int len) {
//ignore, since content length of zipped content
//does not match content length of unzipped content.
}
}
class GZipServletOutputStream extends ServletOutputStream {
private GZIPOutputStream gzipOutputStream = null;
public GZipServletOutputStream(OutputStream output)
throws IOException {
super();
this.gzipOutputStream = new GZIPOutputStream(output);
}
@Override
public void close() throws IOException {
this.gzipOutputStream.close();
}
@Override
public void flush() throws IOException {
this.gzipOutputStream.flush();
}
@Override
public void write(byte b[]) throws IOException {
this.gzipOutputStream.write(b);
}
@Override
public void write(byte b[], int off, int len) throws IOException {
this.gzipOutputStream.write(b, off, len);
}
@Override
public void write(int b) throws IOException {
this.gzipOutputStream.write(b);
}
}
web.xml 配置
为了激活Gzip Servlet Filter，我们需要配置一些东西。
<filter>
<filter-name>GzipFilter</filter-name>
<filter-class>com.xxx.GZipServletFilter</filter-class> #这里填写自己的GzipServletFilter类路径。
</filter>
<filter-mapping>
<filter-name>GzipFilter</filter-name>
<url-pattern>*.js</url-pattern>
</filter-mapping>
<filter-mapping>
<filter-name>GzipFilter</filter-name>
<url-pattern>*.css</url-pattern>
</filter-mapping>
<filter-mapping>
<filter-name>GzipFilter</filter-name>
<url-pattern>*.html</url-pattern>
</filter-mapping>
<filter-mapping>
<filter-name>GzipFilter</filter-name>
<url-pattern>*.jsp</url-pattern>
</filter-mapping>
<filter-mapping>
<filter-name>GzipFilter</filter-name>
<url-pattern>/</url-pattern>
</filter-mapping>
