转 java filter

转自 http://log-cd.javaeye.com/blog/423179#

关键字: filter

    过滤器Filter也具有生命周期：init()->doFilter()->destroy()，由部署文件中的filter元素驱动。在servlet2.4中，过滤器同样可以用于请求分派器，但须在web.xml中声明，<dispatcher>INCLUDE或FORWARD或REQUEST或ERROR</dispatcher>该元素位于filter-mapping中。

一、批量设置请求编码

public class EncodingFilter implements Filter {

    private String encoding = null;

    public void destroy() {
        encoding = null;
    }

    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
        String encoding = getEncoding();
        if (encoding == null){
            encoding = "gb2312";
        }
        request.setCharacterEncoding(encoding);// 在请求里设置上指定的编码
        chain.doFilter(request, response);
    }

    public void init(FilterConfig filterConfig) throws ServletException {
        this.encoding = filterConfig.getInitParameter("encoding");
    }

    private String getEncoding() {
        return this.encoding;
    }

}

public class EncodingFilter implements Filter {

	private String encoding = null;

	public void destroy() {
		encoding = null;
	}

	public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {
		String encoding = getEncoding();
		if (encoding == null){
			encoding = "gb2312";
		}
		request.setCharacterEncoding(encoding);// 在请求里设置上指定的编码
		chain.doFilter(request, response);
	}

	public void init(FilterConfig filterConfig) throws ServletException {
		this.encoding = filterConfig.getInitParameter("encoding");
	}

	private String getEncoding() {
		return this.encoding;
	}

}

<filter>
    <filter-name>EncodingFilter</filter-name>
    <filter-class>com.logcd.filter.EncodingFilter</filter-class>
    <init-param>
       <param-name>encoding</param-name>
       <param-value>gb2312</param-value>
    </init-param>
</filter>

<filter-mapping>
   <filter-name>EncodingFilter</filter-name>
   <url-pattern>/*</url-pattern>
</filter-mapping>

<filter>
    <filter-name>EncodingFilter</filter-name>
    <filter-class>com.logcd.filter.EncodingFilter</filter-class>
    <init-param>
       <param-name>encoding</param-name>
       <param-value>gb2312</param-value>
    </init-param>
</filter>

<filter-mapping>
   <filter-name>EncodingFilter</filter-name>
   <url-pattern>/*</url-pattern>
</filter-mapping>

二、用filter控制用户访问权限

public void doFilter(ServletRequest request,
        ServletResponse response,
        FilterChain chain)
        throws IOException, ServletException {

    HttpServletRequest req = (HttpServletRequest) request;
    HttpServletResponse res = (HttpServletResponse) response;

    HttpSession session = req.getSession();
    if (session.getAttribute("username") != null) {//登录后才能访问
        chain.doFilter(request, response);
    } else {
        res.sendRedirect("../failure.jsp");
    }
}

public void doFilter(ServletRequest request,
        ServletResponse response,
        FilterChain chain)
        throws IOException, ServletException {

    HttpServletRequest req = (HttpServletRequest) request;
    HttpServletResponse res = (HttpServletResponse) response;

    HttpSession session = req.getSession();
    if (session.getAttribute("username") != null) {//登录后才能访问
        chain.doFilter(request, response);
    } else {
        res.sendRedirect("../failure.jsp");
    }
}

<filter>
    <filter-name>SecurityFilter</filter-name>
    <filter-class>com.logcd.filter.SecurityFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>SecurityFilter</filter-name>
    <url-pattern>/admin/*</url-pattern>
</filter-mapping>

<filter>
    <filter-name>SecurityFilter</filter-name>
    <filter-class>com.logcd.filter.SecurityFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>SecurityFilter</filter-name>
    <url-pattern>/admin/*</url-pattern>
</filter-mapping>

三、过滤链

   两个过滤器，EncodingFilter负责设置编码，SecurityFilter负责控制权限，服务器会按照web.xml中过滤器定义的先后循序组装成一条链，然后一次执行其中的doFilter()方法。执行的顺序就如上图所示，执行第一个过滤器的chain.doFilter()之前的代码，第二个过滤器的chain.doFilter()之前的代码，请求的资源，第二个过滤器的chain.doFilter()之后的代码，第一个过滤器的chain.doFilter()之后的代码，最后返回响应。
执行的代码顺序是：

1. 执行EncodingFilter.doFilter()中chain.doFilter()之前的部分：request.setCharacterEncoding("gb2312");
2. 执行SecurityFilter.doFilter()中chain.doFilter()之前的部分：判断用户是否已登录。
3. 如果用户已登录，则访问请求的资源：/admin/index.jsp。
4. 如果用户未登录，则页面重定向到：/failure.jsp。
5. 执行SecurityFilter.doFilter()中chain.doFilter()之后的部分：这里没有代码。
6. 执行EncodingFilter.doFilter()中chain.doFilter()之后的部分：这里也没有代码。

   过滤链的好处是，执行过程中任何时候都可以打断，只要不执行chain.doFilter()就不会再执行后面的过滤器和请求的内容。而在实际使用时，就要特别注意过滤链的执行顺序问题，像EncodingFilter就一定要放在所有Filter之前，这样才能确保在使用请求中的数据前设置正确的编码。

四、使用filter,结合gzip 压缩技术,解决web应用中网络传输数据量大的问题
    gzip是http协议中使用的一种加密算法,客户端向web服务器端发出了请求后，通常情况下服务器端会将页面文件和其他资源，返回到客户端，客户端加载后渲染呈现，这种情况文件一般都比较大，如果开启Gzip ，那么服务器端响应后，会将页面，JS,CSS等文本文件或者其他文件通过高压缩算法将其压缩，然后传输到客户端，由客户端的浏览器负责解压缩与呈现。通常能节省40%以上的流量（一般都有60%左右），一些PHP，JSP文件也能够进行压缩。
1.Tomcat 直接开启Gzip
    打开Tomcat 目录下的conf下的server.xml，并找到如下信息:

<!-- Note : To use gzip compression you could set the following properties :
   compression="on"
   compressionMinSize="2048"
   noCompressionUserAgents="gozilla, traviata"
   compressableMimeType="text/html,text/xml"
-->

<!-- Note : To use gzip compression you could set the following properties :
   compression="on" 
   compressionMinSize="2048" 
   noCompressionUserAgents="gozilla, traviata" 
   compressableMimeType="text/html,text/xml"
-->

    把它们加入到你配置的<Connector port="80" .../>中去。如果要压缩css 和 js，加入compressableMimeType="text/html,text/xml,text/css,text/javascript"。还要压缩图片,加入compressableMimeType="text/html,text/xml,text/css,text/javascript,image/gif,image/jpg"。
    开启后重启Tomcat ，通过浏览器查看headers信息就能看到是否开启。

2.使用filter,在代码级别完成web应用的gzip压缩的开启。

(1).CachedResponseWrapper类
    实现定制输出的关键是对HttpServletResponse 进行包装，截获所有的输出，等到过滤器链处理完毕后，再对截获的输出进行处理，并写入到真正的HttpServletResponse 对象中。JavaEE 框架已经定义了一个HttpServletResponseWrapper 类使得包装HttpServletResponse 更加容易。我们扩展这个HttpServletResponseWrapper，截获所有的输出，并保存到ByteArrayOutputStream 中。
    定制的包装响应能方便地从帮助类 HttpServletResponseWrapper 中导出。这一类粗略地执行许多方法，允许我们简单地覆盖 getOutputStream() 方法以及 getWriter() 方法，提供了定制输出流的实例。
    HttpServletResponseWrapper这个类的使用包括以下五个步骤：
1）建立一个响应包装器。扩展javax.servlet.http.HttpServletResponseWrapper。
2）提供一个缓存输出的PrintWriter。重载getWriter方法，返回一个保存发送给它的所有东西的PrintWriter，并把结果存进一个可以稍后访问的字段中。
3）传递该包装器给doFilter。此调用是合法的，因为HttpServletResponseWrapper实现HttpServletResponse。
4）提取和修改输出。在调用FilterChain的doFilter方法后，原资源的输出只要利用步骤2中提供的机制就可以得到。只要对你的应用适合，就可以修改或替换它。
5）发送修改过的输出到客户机。因为原资源不再发送输出到客户机（这些输出已经存放到你的响应包装器中了），所以必须发送这些输出。这样，你的过滤器需要从原响应对象中获得PrintWriter或OutputStream，并传递修改过的输出到该流中。

/**
 * Wrapper:在内存中开辟一个ByteOutputStream，然后将拦截的响应写入byte[]，
 * 写入完毕后，再将wrapper的byte[]写入真正的response对象
 * This class is used for wrapped response for getting cached data.
 */
class CachedResponseWrapper extends HttpServletResponseWrapper {

    /**
     * Indicate that getOutputStream() or getWriter() is not called yet.
     */
    public static final int OUTPUT_NONE = 0;

    /**
     * Indicate that getWriter() is already called.
     */
    public static final int OUTPUT_WRITER = 1;

    /**
     * Indicate that getOutputStream() is already called.
     */
    public static final int OUTPUT_STREAM = 2;

    private int outputType = OUTPUT_NONE;

    private int status = SC_OK;

    private ServletOutputStream output = null;

    private PrintWriter writer = null;

    private ByteArrayOutputStream buffer = null;

    public CachedResponseWrapper(HttpServletResponse resp) throws IOException {
        super(resp);
        buffer = new ByteArrayOutputStream();
    }

    public int getStatus() {
        return status;
    }

    public void setStatus(int status) {
        super.setStatus(status);
        this.status = status;
    }

    public void setStatus(int status, String string) {
        super.setStatus(status, string);
        this.status = status;
    }

    public void sendError(int status, String string) throws IOException {
        super.sendError(status, string);
        this.status = status;
    }

    public void sendError(int status) throws IOException {
        super.sendError(status);
        this.status = status;
    }

    public void sendRedirect(String location) throws IOException {
        super.sendRedirect(location);
        this.status = SC_MOVED_TEMPORARILY;
    }

    public PrintWriter getWriter() throws IOException {
        if (outputType == OUTPUT_STREAM)
            throw new IllegalStateException();
        else if (outputType == OUTPUT_WRITER)
            return writer;
        else {
            outputType = OUTPUT_WRITER;
            writer = new PrintWriter(new OutputStreamWriter(buffer,
                    getCharacterEncoding()));
            return writer;
        }
    }

    public ServletOutputStream getOutputStream() throws IOException {
        if (outputType == OUTPUT_WRITER)
            throw new IllegalStateException();
        else if (outputType == OUTPUT_STREAM)
            return output;
        else {
            outputType = OUTPUT_STREAM;
            output = new WrappedOutputStream(buffer);
            return output;
        }
    }

    public void flushBuffer() throws IOException {
        if (outputType == OUTPUT_WRITER)
            writer.flush();
        if (outputType == OUTPUT_STREAM)
            output.flush();
    }

    public void reset() {
        outputType = OUTPUT_NONE;
        buffer.reset();
    }

    /**
     * Call this method to get cached response data.
     *
     * @return byte array buffer.
     * @throws IOException
     */
    public byte[] getResponseData() throws IOException {
        flushBuffer();
        return buffer.toByteArray();
    }

    /**
     * This class is used to wrap a ServletOutputStream and store output stream
     * in byte[] buffer.
     */
    class WrappedOutputStream extends ServletOutputStream {

        private ByteArrayOutputStream buffer;

        public WrappedOutputStream(ByteArrayOutputStream buffer) {
            this.buffer = buffer;
        }

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

        public byte[] toByteArray() {
            return buffer.toByteArray();
        }
    }

}

/**
 * Wrapper:在内存中开辟一个ByteOutputStream，然后将拦截的响应写入byte[]，
 * 写入完毕后，再将wrapper的byte[]写入真正的response对象
 * This class is used for wrapped response for getting cached data.
 */ 
class CachedResponseWrapper extends HttpServletResponseWrapper {

	/**
	 * Indicate that getOutputStream() or getWriter() is not called yet.
	 */
	public static final int OUTPUT_NONE = 0;

	/**
	 * Indicate that getWriter() is already called.
	 */
	public static final int OUTPUT_WRITER = 1;

	/**
	 * Indicate that getOutputStream() is already called.
	 */
	public static final int OUTPUT_STREAM = 2;

	private int outputType = OUTPUT_NONE;

	private int status = SC_OK;

	private ServletOutputStream output = null;

	private PrintWriter writer = null;

	private ByteArrayOutputStream buffer = null;

	public CachedResponseWrapper(HttpServletResponse resp) throws IOException {
		super(resp);
		buffer = new ByteArrayOutputStream();
	}

	public int getStatus() {
		return status;
	}

	public void setStatus(int status) {
		super.setStatus(status);
		this.status = status;
	}

	public void setStatus(int status, String string) {
		super.setStatus(status, string);
		this.status = status;
	}

	public void sendError(int status, String string) throws IOException {
		super.sendError(status, string);
		this.status = status;
	}

	public void sendError(int status) throws IOException {
		super.sendError(status);
		this.status = status;
	}

	public void sendRedirect(String location) throws IOException {
		super.sendRedirect(location);
		this.status = SC_MOVED_TEMPORARILY;
	}

	public PrintWriter getWriter() throws IOException {
		if (outputType == OUTPUT_STREAM)
			throw new IllegalStateException();
		else if (outputType == OUTPUT_WRITER)
			return writer;
		else {
			outputType = OUTPUT_WRITER;
			writer = new PrintWriter(new OutputStreamWriter(buffer,
					getCharacterEncoding()));
			return writer;
		}
	}

	public ServletOutputStream getOutputStream() throws IOException {
		if (outputType == OUTPUT_WRITER)
			throw new IllegalStateException();
		else if (outputType == OUTPUT_STREAM)
			return output;
		else {
			outputType = OUTPUT_STREAM;
			output = new WrappedOutputStream(buffer);
			return output;
		}
	}

	public void flushBuffer() throws IOException {
		if (outputType == OUTPUT_WRITER)
			writer.flush();
		if (outputType == OUTPUT_STREAM)
			output.flush();
	}

	public void reset() {
		outputType = OUTPUT_NONE;
		buffer.reset();
	}

	/**
	 * Call this method to get cached response data.
	 * 
	 * @return byte array buffer.
	 * @throws IOException
	 */
	public byte[] getResponseData() throws IOException {
		flushBuffer();
		return buffer.toByteArray();
	}

	/**
	 * This class is used to wrap a ServletOutputStream and store output stream
	 * in byte[] buffer.
	 */
	class WrappedOutputStream extends ServletOutputStream {

		private ByteArrayOutputStream buffer;

		public WrappedOutputStream(ByteArrayOutputStream buffer) {
			this.buffer = buffer;
		}

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

		public byte[] toByteArray() {
			return buffer.toByteArray();
		}
	}

}

(2).GZipFilter类

public class GZipFilter implements Filter {
    public void init(FilterConfig arg0) throws ServletException {
    }

    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {

        HttpServletResponse httpResponse = (HttpServletResponse) response;
        CachedResponseWrapper wrapper = new CachedResponseWrapper(httpResponse);
        // 写入wrapper:
        chain.doFilter(request, wrapper);
        // 对响应进行处理，这里是进行GZip压缩:
        byte[] data = GZipUtil.gzip(wrapper.getResponseData());
        httpResponse.setHeader("Content-Encoding", "gzip");
        httpResponse.setContentLength(data.length);
        ServletOutputStream output = response.getOutputStream();
        output.write(data);
        output.flush();
    }

    public void destroy() {

    }

}

public class GZipFilter implements Filter {
	public void init(FilterConfig arg0) throws ServletException {
	}

	public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {

		HttpServletResponse httpResponse = (HttpServletResponse) response;
		CachedResponseWrapper wrapper = new CachedResponseWrapper(httpResponse);
		// 写入wrapper:
		chain.doFilter(request, wrapper);
		// 对响应进行处理，这里是进行GZip压缩:
		byte[] data = GZipUtil.gzip(wrapper.getResponseData());
		httpResponse.setHeader("Content-Encoding", "gzip");
		httpResponse.setContentLength(data.length);
		ServletOutputStream output = response.getOutputStream();
		output.write(data);
		output.flush();
	}

	public void destroy() {

	}

}

(3).GZipUtil类

public final class GZipUtil {
    /** * Do a gzip operation. */
    public static byte[] gzip(byte[] data) {
        ByteArrayOutputStream byteOutput = new ByteArrayOutputStream(10240);
        GZIPOutputStream output = null;
        try {
            output = new GZIPOutputStream(byteOutput);
            output.write(data);
        } catch (IOException e) {
            throw new RuntimeException("G-Zip failed.", e);
        } finally {
            if (output != null) {
                try {
                    output.close();
                } catch (IOException e) {
                }
            }
        }
        return byteOutput.toByteArray();
    }
}

public final class GZipUtil {
	/** * Do a gzip operation. */
	public static byte[] gzip(byte[] data) {
		ByteArrayOutputStream byteOutput = new ByteArrayOutputStream(10240);
		GZIPOutputStream output = null;
		try {
			output = new GZIPOutputStream(byteOutput);
			output.write(data);
		} catch (IOException e) {
			throw new RuntimeException("G-Zip failed.", e);
		} finally {
			if (output != null) {
				try {
					output.close();
				} catch (IOException e) {
				}
			}
		}
		return byteOutput.toByteArray();
	}
}

(4).在web.xml中配置 GZipFilter

<filter>
   <filter-name>GZipFilter</filter-name>
   <filter-class>com.logcd.filter.GZipFilter</filter-class>
</filter>
<filter-mapping>
   <filter-name>GZipFilter</filter-name>
   <url-pattern>*.html</url-pattern>
</filter-mapping>