Decorator设计模式在Servlet应用中的实践应用

Decorator设计模式（装饰模式）是一种常用的设计模式，其核心思想是在不修改原有类文件的情况下，通过包装对象动态地扩展对象的功能。这种模式在Java web开发中有着广泛的应用，尤其是在Servlet应用中，对request和response对象进行功能增强。

Decorator设计模式介绍

Decorator模式的主要思想是通过创建一个包装对象（Decorator），来对目标对象（Target）进行功能增强。这种模式的主要优势在于：

无需继承：无需对目标对象的类进行继承，避免了类层次结构复杂化。

动态扩展：可以在运行时动态地对目标对象的功能进行扩展，而不需要重新编译目标类。

透明性：对客户端来说，Decorator对象与目标对象看起来是完全透明的，无需额外的接口定义。

在Servlet开发中，request和response对象是由容器动态创建的，其具体实现类并非公开的。因此，为了对这些对象进行功能增强，开发人员通常会采用Decorator模式对其进行包装。

request对象的功能增强

中文乱码处理

过滤器实现

编写一个用于解决中文乱码的过滤器 CharacterEncodingFilter。该过滤器的主要功能是确保不同请求方式（如GET、POST）下的参数能够正确编码和解码。

package me.gacl.web.filter;import java.io.IOException;import javax.servlet.Filter;import javax.servlet.FilterChain;import javax.servlet.FilterConfig;import javax.servlet.ServletException;import javax.servlet.ServletRequest;import javax.servlet.ServletResponse;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletRequestWrapper;public class CharacterEncodingFilter implements Filter {    private FilterConfig filterConfig;    private String defaultCharset = "UTF-8";    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException, ServletException {        HttpServletRequest request = (HttpServletRequest) req;        HttpServletResponse response = (HttpServletResponse) resp;        String charset = filterConfig.getInitParameter("charset");        if (charset == null) {            charset = defaultCharset;        }        request.setCharacterEncoding(charset);        response.setCharacterEncoding(charset);        response.setContentType("text/html;charset=" + charset);        MyCharacterEncodingRequest requestWrapper = new MyCharacterEncodingRequest(request);        chain.doFilter(requestWrapper, response);    }    public void init(FilterConfig filterConfig) throws ServletException {        this.filterConfig = filterConfig;    }}class MyCharacterEncodingRequest extends HttpServletRequestWrapper {    private HttpServletRequest request;    public MyCharacterEncodingRequest(HttpServletRequest request) {        super(request);        this.request = request;    }    @Override    public String getParameter(String name) {        String value = this.request.getParameter(name);        if (value == null) {            return null;        }        if (!this.request.getMethod().equalsIgnoreCase("get")) {            return value;        } else {            value = new String(value.getBytes("ISO-8859-1"), this.request.getCharacterEncoding());            return value;        }    }}

配置和使用

在 web.xml 中配置过滤器：


       
    
     CharacterEncodingFilter
        
    
     me.gacl.web.filter.CharacterEncodingFilter
    
   
       
    
     CharacterEncodingFilter
        
    
     /*

HTML标签转义

编写一个用于转义HTML标签的过滤器 HtmlFilter。该过滤器会对请求参数中的HTML标签进行转义处理。

package me.gacl.web.filter;import java.io.IOException;import javax.servlet.Filter;import javax.servlet.FilterChain;import javax.servlet.FilterConfig;import javax.servlet.ServletException;import javax.servlet.ServletRequest;import javax.servlet.ServletResponse;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletRequestWrapper;public class HtmlFilter implements Filter {    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException, ServletException {        HttpServletRequest request = (HttpServletRequest) req;        HttpServletResponse response = (HttpServletResponse) resp;        MyHtmlRequest requestWrapper = new MyHtmlRequest(request);        chain.doFilter(requestWrapper, response);    }}class MyHtmlRequest extends HttpServletRequestWrapper {    private HttpServletRequest request;    public MyHtmlRequest(HttpServletRequest request) {        super(request);        this.request = request;    }    @Override    public String getParameter(String name) {        String value = this.request.getParameter(name);        if (value == null) {            return null;        }        return filter(value);    }    public String filter(String message) {        if (message == null) {            return null;        }        StringBuilder sb = new StringBuilder(message.length() + 50);        for (int i = 0; i < message.length(); i++) {            switch (message.charAt(i)) {                case '&':                    sb.append("&");                    break;                case '<':                    sb.append("<");                    break;                case '>':                    sb.append(">");                    break;                case '"':                    sb.append(""");                    break;                default:                    sb.append(message.charAt(i));            }        }        return sb.toString();    }}

敏感字符过滤

编写一个用于过滤敏感字符的过滤器 DirtyFilter。该过滤器会对请求参数中的敏感词进行替换。

package me.gacl.web.filter;import java.io.BufferedReader;import java.io.IOException;import java.io.InputStream;import java.io.InputStreamReader;import java.util.ArrayList;import java.util.List;import javax.servlet.Filter;import javax.servlet.FilterChain;import javax.servlet.FilterConfig;import javax.servlet.ServletException;import javax.servlet.ServletRequest;import javax.servlet.ServletResponse;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletRequestWrapper;public class DirtyFilter implements Filter {    private FilterConfig filterConfig;    private List
   
     dirtyWords;    @Override    public void init(FilterConfig filterConfig) throws ServletException {        this.filterConfig = filterConfig;        dirtyWords = new ArrayList<>();        String dirtyWordPath = filterConfig.getInitParameter("dirtyWord");        InputStream inputStream = filterConfig.getServletContext().getResourceAsStream(dirtyWordPath);        if (inputStream != null) {            try {                InputStreamReader is = new InputStreamReader(inputStream, "UTF-8");                BufferedReader reader = new BufferedReader(is);                String line;                while ((line = reader.readLine()) != null) {                    dirtyWords.add(line.trim());                }            } catch (IOException e) {                e.printStackTrace();            }        }    }    @Override    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException, ServletException {        HttpServletRequest request = (HttpServletRequest) req;        HttpServletResponse response = (HttpServletResponse) resp;        DirtyRequest dirtyRequest = new DirtyRequest(request);        chain.doFilter(dirtyRequest, response);    }    class DirtyRequest extends HttpServletRequestWrapper {        private HttpServletRequest request;        public DirtyRequest(HttpServletRequest request) {            super(request);            this.request = request;        }        @Override        public String getParameter(String name) {            String value = this.request.getParameter(name);            if (value == null) {                return null;            }            for (String dirtyWord : dirtyWords) {                if (value.contains(dirtyWord)) {                    System.out.println("发现敏感词：" + dirtyWord + ", 已替换为****");                    value = value.replace(dirtyWord, "****");                }            }            return value;        }    }}

response对象的功能增强

响应内容压缩

编写一个用于对响应内容进行GZIP压缩的过滤器 GzipFilter。该过滤器会对响应内容进行压缩处理。

package me.gacl.web.filter;import java.io.ByteArrayOutputStream;import java.io.IOException;import java.io OutputStreamWriter;import java.io PrintWriter;import java.util.zip.GZIPOutputStream;import javax.servlet.Filter;import javax.servlet.FilterChain;import javax.servlet.FilterConfig;import javax.servlet.ServletException;import javax.servlet.ServletOutputStream;import javax.servlet.ServletRequest;import javax.servlet.ServletResponse;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpServletResponseWrapper;public class GzipFilter implements Filter {    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException {        HttpServletRequest request = (HttpServletRequest) req;        HttpServletResponse response = (HttpServletResponse) resp;        BufferResponse myResponse = new BufferResponse(response);        chain.doFilter(request, myResponse);        byte[] compressedContent = myResponse.getBuffer();        System.out.println("压缩前大小：" + compressedContent.length);        ByteArrayOutputStream bout = new ByteArrayOutputStream();        GZIPOutputStream gzip = new GZIPOutputStream(bout);        gzip.write(compressedContent);        gzip.close();        byte[] gzipContent = bout.toByteArray();        System.out.println("压缩后大小：" + gzipContent.length);        response.setHeader("Content-Encoding", "gzip");        response.setContentLength(gzipContent.length);        response.getOutputStream().write(gzipContent);    }    class BufferResponse extends HttpServletResponseWrapper {        private ByteArrayOutputStream bout;        private PrintWriter pw;        public BufferResponse(HttpServletResponse response) {            super(response);        }        @Override        public ServletOutputStream getOutputStream() {            return new MyServletOutputStream(bout);        }        @Override        public PrintWriter getWriter() {            return new PrintWriter(new OutputStreamWriter(bout, response.getCharacterEncoding()));        }        public byte[] getBuffer() {            try {                if (pw != null) {                    pw.close();                }                if (bout != null) {                    bout.flush();                    return bout.toByteArray();                }                return null;            } catch (Exception e) {                throw new RuntimeException(e);            }        }    }    class MyServletOutputStream extends ServletOutputStream {        private ByteArrayOutputStream bout;        public MyServletOutputStream(ByteArrayOutputStream bout) {            this.bout = bout;        }        @Override        public void write(int b) throws IOException {            this.bout.write(b);        }    }}

数据缓存

编写一个用于将Web资源缓存到内存的过滤器 WebResourceCachedFilter。该过滤器会对频繁访问的页面内容进行缓存处理。

package me.gacl.web.filter;import java.io.ByteArrayOutputStream;import java.io.IOException;import java.io OutputStreamWriter;import java.io PrintWriter;import java.util.HashMap;import java.util.Map;import javax.servlet.Filter;import javax.servlet.FilterChain;import javax.servlet.FilterConfig;import javax.servlet.ServletException;import javax.servlet.ServletOutputStream;import javax.servlet.ServletRequest;import javax.servlet.ServletResponse;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpServletResponseWrapper;public class WebResourceCachedFilter implements Filter {    private Map
   
     resourceCache;    public void init(FilterConfig filterConfig) throws ServletException {    }    @Override    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException {        HttpServletRequest request = (HttpServletRequest) req;        HttpServletResponse response = (HttpServletResponse) resp;        String uri = request.getRequestURI();        byte[] cachedContent = resourceCache.get(uri);        if (cachedContent != null) {            response.getOutputStream().write(cachedContent);            return;        }        BufferResponse myResponse = new BufferResponse(response);        chain.doFilter(request, myResponse);        byte[] content = myResponse.getBuffer();        resourceCache.put(uri, content);        response.getOutputStream().write(content);    }    class BufferResponse extends HttpServletResponseWrapper {        private ByteArrayOutputStream bout;        private PrintWriter pw;        public BufferResponse(HttpServletResponse response) {            super(response);        }        @Override        public ServletOutputStream getOutputStream() {            return new MyServletOutputStream(bout);        }        @Override        public PrintWriter getWriter() {            return new PrintWriter(new OutputStreamWriter(bout, response.getCharacterEncoding()));        }        public byte[] getBuffer() {            try {                if (pw != null) {                    pw.close();                }                return bout.toByteArray();            } catch (Exception e) {                throw new RuntimeException(e);            }        }    }    class MyServletOutputStream extends ServletOutputStream {        private ByteArrayOutputStream bout;        public MyServletOutputStream(ByteArrayOutputStream bout) {            this.bout = bout;        }        @Override        public void write(int b) throws IOException {            this.bout.write(b);        }    }}

总结

通过以上实例可以看出，Decorator设计模式在Servlet开发中具有广泛的应用场景。无论是对request对象的参数处理，还是对response对象的内容输出，都可以通过包装目标对象来实现功能的增强。在实际项目中，可以根据具体需求选择性地对request和response对象进行装饰，以满足特定的功能需求。

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