SpringBoot中filter的使用详解及原理

首先还是老生常谈，我先把SpringBoot中filter的使用示例写出来，然后再解释下代码、说一下运行的顺序，最后讲一下filter的原理（其实就是责任链设计模式，从马士兵老师那里偷来的。。。）。

要想使用filter，需要写一个方法继承Filter类，我们写如下两个自己的Filter类，首先是FirstFilter类，其中@Order里边的数字越小代表越先被该Filter过滤，@WebFilter代表这是个Filter类并把这个类注入到容器中：

package com.example.executor_test.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.annotation.WebFilter;

import org.springframework.core.annotation.Order;

@Order(1)
@WebFilter(filterName="firstFilter", urlPatterns="/*")
public class FirstFilter implements Filter {

	@Override
	public void init(FilterConfig filterConfig) throws ServletException {
		
	}

	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {
		System.out.println("first filter 1");
		chain.doFilter(request, response);
		System.out.println("first filter 2");
	}

	@Override
	public void destroy() {
		
	}
}

然后是第二个Filter，SecondFilter类：

package com.example.executor_test.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.annotation.WebFilter;

import org.springframework.core.annotation.Order;

@Order(2)
@WebFilter(filterName="secondFilter", urlPatterns="/*")
public class SecondFilter implements Filter {

	@Override
	public void init(FilterConfig filterConfig) throws ServletException {

	}

	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {
		System.out.println("second filter 1");
		System.out.println("before:" + response);
		chain.doFilter(request, response);
		System.out.println("after:" + response);
		System.out.println("second filter 2");
		
	}

	@Override
	public void destroy() {
		
	}
}

然后我们把Controller类也写出来吧：

package com.example.executor_test.controller;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;

import javax.annotation.Resource;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

import com.example.executor_test.task.OldTask;
import com.example.executor_test.task.OldTaskThread;

@RestController
public class TestController {
	
	@GetMapping("/test1")
	public String test1() {
		System.out.println("method in controller");
		return "test1";
	}

}

最后是springboot的主方法入口，注意，由于我们使用注解注入的Filter，所以要在下边这个Application类中加入@ServletComponentScan注解：

package com.example.executor_test;

import org.omg.CORBA.PRIVATE_MEMBER;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.servlet.ServletComponentScan;
import org.springframework.context.ConfigurableApplicationContext;

import com.example.executor_test.task.OldTaskThread;

@SpringBootApplication
@ServletComponentScan
public class ExecutorTestApplication {

	public static void main(String[] args) {	
		ConfigurableApplicationContext applicationContext = SpringApplication.run(ExecutorTestApplication.class, args);
	}
}

首先我们先来看一下执行结果，启动后访问127.0.0.1:8080/test1，在后台中打印如下信息：

我们可以看出代码执行的流程，首先请求被firstfilter截获，打印出first filter 1，然后去执行chain.doFilter(request, response)，这句话代表着请求会转发给过滤器链上下一个对象，也就是secondfilter，所以打印出secondfilter里的second filter 1，接下来再执行secondfilter里的chain.dofilter()方法，请求再转发给下一个对象，由于没有其他的filter了，所以会转发给controller，打印出了controller类中的method in controller，接下来再去内存栈里调用secondfilter的print("second filter 2")，然后再去内存栈里调用firstfilter的print("first filter 1")。所以如果在自己实现的Filter类的doFilter方法里不加chain.doFilter(req, rep)是万万不行的，那样会导致请求到了这个filter里就不再往下走了，永远进不了controller中。

我们也可以在print("before:" + response)和print("after:" + response)这两个地方打上断点，然后调试一下，你会发现在before那里的response里是什么都么有的，而在after那里的response里则是已经有了test1字符串，也就是说controller类test1方法的返回值已经添加进了response，所以如果你想对请求的response做一下过滤处理，那么一定要在chain.doFilter(res, rep)之后写你的逻辑。

接下来讲一下这个Filter和FilterChain都是怎么用责任链模式实现的，如果不太需要了解原理的话，往下的部分就可以不看了。。。好了，我们来模拟一下简单的实现SpringBoot中的Filter接口和FilterChain类：

首先是我们自己写的Filter接口，里边就一个doFilter方法：

package filterchain_pattern;

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

}

接下来是我们自己写的FilterChain类：

package filterchain_pattern;

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

public class FilterChain implements Filter {

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

}

接下来模拟Request类和Response类：

package filterchain_pattern;

public class Request {
	
	public String requestStr;

}

package filterchain_pattern;

public class Response {

	public String responseStr;
	
}

然后我们下一个Filter接口的实现类HTMLFilter类，该类会将requestStr中的<>替换成[]，并给responseStr添加------------HTML response filter字符串，并在控制台打印出来：

package filterchain_pattern;

public class HTMLFilter implements Filter {

	@Override
	public void doFilter(Request request, Response response, FilterChain chain) {
		request.requestStr = request.requestStr.replace("<", "[").replace(">", "]") + "--------HTML Request Filter";
		System.out.println("HTML Filter request Str:" + request.requestStr);
		chain.doFilter(request, response, chain);
		response.responseStr = response.responseStr + "-------------HTML response filter";
		System.out.println("HTML Filter response Str:" + response.responseStr);
	}
}

然后是另外一个Filter接口的实现类SensitiveFilter类，该类会给requestStr添加一段字符串，给responseStr添加一段字符串，并在控制台打印出来：

package filterchain_pattern;

public class SensitiveFilter implements Filter {

	@Override
	public void doFilter(Request request, Response response, FilterChain chain) {
		request.requestStr = request.requestStr + "---------------Sensitive request Filter";
		System.out.println("sensitiveFilter request str:" + request.requestStr);
		chain.doFilter(request, response, chain);
		response.responseStr = response.responseStr + "---------------------sensitive response filter";
		System.out.println("sensitiveFilter response str:" + response.responseStr);
	}
	
}

最后使我们的Main方法类：

package filterchain_pattern;

public class MainTest {

	public static void main(String[] args) {
		
		String msg = "<html>testMsg</html>";
		Request request = new Request();
		request.requestStr = msg;
		Response response = new Response();
		response.responseStr = "responseStr";
		
		FilterChain fc = new FilterChain();
		fc.addFilter(new HTMLFilter()).addFilter(new SensitiveFilter());
		fc.doFilter(request, response, fc);

	}

}

打印结果如下，这就是责任链模式的实际应用了：

SpringBoot中filter的使用详解及原理

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