java 之 编码实现Filter学习示例

public interface Filter {
void doFilter(Request request,Response response,FilterChain chain);
}

public class HTMLFilter implements Filter {

@Override
public void doFilter(Request request, Response response, FilterChain chain) {
request.setRequestStr(request.getRequestStr().replace('<', '[').replace('>', ']'));
chain.doFilter(request, response, chain);
response.setResponseStr("-----------HtmlFilter()");
}

}

public class SensitiveFilter implements Filter {

@Override
public void doFilter(Request request, Response response, FilterChain chain) {
request.setRequestStr(request.getRequestStr().replace("s","js"));
chain.doFilter(request, response, chain);
response.setResponseStr("-----------SensitiveFilter()");
}

}

import java.util.ArrayList;
import java.util.List;

public class FilterChain implements Filter {
List<Filter> filters = new ArrayList<Filter>();
int index = 0;
public FilterChain addFilter(Filter f) {
this.filters.add(f);
return this;
}
@Override
public void doFilter(Request request, Response response, FilterChain chain) {
if(index==filters.size()) {
return;
}

Filter f = filters.get(index);
index++;

f.doFilter(request, response, chain);
}

}

public class Request {
private String requestStr;

public String getRequestStr() {
return requestStr;
}

public void setRequestStr(String requestStr) {
this.requestStr = requestStr;
}
}

public class Response {
private String responseStr="response";

public String getResponseStr() {
return responseStr;
}

public void setResponseStr(String responseStr) {
this.responseStr += responseStr;
}
}

public class TestFilter {

/**
* @param args
*/
public static void main(String[] args) {
String msg = "<script>";
Request request = new Request();
request.setRequestStr(msg);

Response response = new Response();

FilterChain fc = new FilterChain();
fc.addFilter(new HTMLFilter()).addFilter(new SensitiveFilter());

fc.doFilter(request, response, fc);

System.out.println(request.getRequestStr());
System.out.println(response.getResponseStr());
}

}

打印结果：

[jscript]

response-----------SensitiveFilter()-----------HtmlFilter()

##########################################################################################

通常我们所访问的资源是一个servlet或jsp页面，而jsp其实是一个被封装了的servlet（每个jsp执行前都会被转化为一个标准的servlet，这点若还有不明白的请自己到网上查一下吧），于是我们就可以统一地认为我们每次访问的都是一个Servlet,而每当我们访问一个servlet时，web容器都会调用该Servlet的service方法去处理请求。而在service方法又会根据请求方式的不同（Get/Post）去调用相应的doGet()或doPost()方法，实际处理请求的就是这个doGet或doPost方法。写过servlet的朋友都应该知道，我们在doGet（或doPost）方法中是通过response.getWriter（）得到客户端的输出流对象，然后用此对象对客户进行响应。
到这里我们就应该理解了过滤器的执行流程了：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——>……——>第n个过滤器的chain.doFilter()之前的代码——>所请求servlet的service()方法中的代码——>所请求servlet的doGet()或doPost()方法中的代码——>第n个过滤器的chain.doFilter()之后的代码——>……——>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。