spring security filter的工作原理

邢飞雨
2023-12-01

这篇文章介绍filter的工作原理。配置方式为xml。

Filter如何进入执行逻辑的

初始配置:

 <filter>
    <filter-name>springSecurityFilterChain</filter-name>
    <filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
  </filter>

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

DelegatingFilterProxy这个类继承了GenericFilterBean,GenericFilterBean实现了Filter接口。

这个配置是一切的开始,配置完这个之后,在启动项目的时候会执行Filterd的初始化方法:

@Override
	public final void init(FilterConfig filterConfig) throws ServletException {
		Assert.notNull(filterConfig, "FilterConfig must not be null");
		if (logger.isDebugEnabled()) {
			logger.debug("Initializing filter '" + filterConfig.getFilterName() + "'");
		}

		this.filterConfig = filterConfig;

		// Set bean properties from init parameters.
		PropertyValues pvs = new FilterConfigPropertyValues(filterConfig, this.requiredProperties);
		if (!pvs.isEmpty()) {
			try {
				BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
				ResourceLoader resourceLoader = new ServletContextResourceLoader(filterConfig.getServletContext());
				Environment env = this.environment;
				if (env == null) {
					env = new StandardServletEnvironment();
				}
				bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, env));
				initBeanWrapper(bw);
				bw.setPropertyValues(pvs, true);
			}
			catch (BeansException ex) {
				String msg = "Failed to set bean properties on filter '" +
					filterConfig.getFilterName() + "': " + ex.getMessage();
				logger.error(msg, ex);
				throw new NestedServletException(msg, ex);
			}
		}

		// Let subclasses do whatever initialization they like.
		initFilterBean(); // 这个方法

		if (logger.isDebugEnabled()) {
			logger.debug("Filter '" + filterConfig.getFilterName() + "' configured successfully");
		}
	}

在初始化方法中,会执行初始化Filter的方法initFilterBean。这个方法的实现在DelegatingFilterProxy中:

protected void initFilterBean() throws ServletException {
		synchronized (this.delegateMonitor) {
			if (this.delegate == null) {
				// If no target bean name specified, use filter name.
				if (this.targetBeanName == null) {
					this.targetBeanName = getFilterName();
				}
				// Fetch Spring root application context and initialize the delegate early,
				// if possible. If the root application context will be started after this
				// filter proxy, we'll have to resort to lazy initialization.
				WebApplicationContext wac = findWebApplicationContext();
				if (wac != null) {
					this.delegate = initDelegate(wac); //这个方法
				}
			}
		}
	}

在这个初始化方法中又调用initDelegate方法进行初始化:

protected Filter initDelegate(WebApplicationContext wac) throws ServletException {
		String targetBeanName = getTargetBeanName();
		Assert.state(targetBeanName != null, "No target bean name set");
		Filter delegate = wac.getBean(targetBeanName, Filter.class);
		if (isTargetFilterLifecycle()) {
			delegate.init(getFilterConfig());
		}
		return delegate;
	}

在这个方法中,先获取targetBeanName,这个名字是构造方法中赋值的:

public DelegatingFilterProxy(String targetBeanName, @Nullable WebApplicationContext wac) {
		Assert.hasText(targetBeanName, "Target Filter bean name must not be null or empty");
		this.setTargetBeanName(targetBeanName);
		this.webApplicationContext = wac;
		if (wac != null) {
			this.setEnvironment(wac.getEnvironment());
		}
	}

这个名字就是web.xml中配置的名字springSecurityFilterChain:

 <filter>
    <filter-name>springSecurityFilterChain</filter-name>
    <filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
  </filter>

springSecurityFilterChain是固定不能改的,如果改了启动时就会报错,这是spring 启动时内置的一个bean,这个bean实际是FilterChainProxy。

这样一个Filter就初始化话好了,过滤器chain也初始化好了。

当一个请求进来的时候,会进入FilterChainProxy执行doFilter方法:

public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {
		boolean clearContext = request.getAttribute(FILTER_APPLIED) == null;
		if (clearContext) {
			try {
				request.setAttribute(FILTER_APPLIED, Boolean.TRUE);
				doFilterInternal(request, response, chain);
			}
			finally {
				SecurityContextHolder.clearContext();
				request.removeAttribute(FILTER_APPLIED);
			}
		}
		else {
			doFilterInternal(request, response, chain);
		}
	}

先获取所有的Filter,然后执行doFilterInternal方法:

private void doFilterInternal(ServletRequest request, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {

		FirewalledRequest fwRequest = firewall
				.getFirewalledRequest((HttpServletRequest) request);
		HttpServletResponse fwResponse = firewall
				.getFirewalledResponse((HttpServletResponse) response);

		List<Filter> filters = getFilters(fwRequest);

		if (filters == null || filters.size() == 0) {
			if (logger.isDebugEnabled()) {
				logger.debug(UrlUtils.buildRequestUrl(fwRequest)
						+ (filters == null ? " has no matching filters"
								: " has an empty filter list"));
			}

			fwRequest.reset();

			chain.doFilter(fwRequest, fwResponse);

			return;
		}

        // 最终执行下面的这些代码
		VirtualFilterChain vfc = new VirtualFilterChain(fwRequest, chain, filters);
		vfc.doFilter(fwRequest, fwResponse);
	}

VirtualFilterChain是一个匿名内部类:

private static class VirtualFilterChain implements FilterChain {
		private final FilterChain originalChain;
		private final List<Filter> additionalFilters;
		private final FirewalledRequest firewalledRequest;
		private final int size;
		private int currentPosition = 0;

		private VirtualFilterChain(FirewalledRequest firewalledRequest,
				FilterChain chain, List<Filter> additionalFilters) {
			this.originalChain = chain;
			this.additionalFilters = additionalFilters;
			this.size = additionalFilters.size();
			this.firewalledRequest = firewalledRequest;
		}

		@Override
		public void doFilter(ServletRequest request, ServletResponse response)
				throws IOException, ServletException {
			if (currentPosition == size) {
				if (logger.isDebugEnabled()) {
					logger.debug(UrlUtils.buildRequestUrl(firewalledRequest)
							+ " reached end of additional filter chain; proceeding with original chain");
				}

				// Deactivate path stripping as we exit the security filter chain
				this.firewalledRequest.reset();

				originalChain.doFilter(request, response);
			}
			else {
				currentPosition++;

				Filter nextFilter = additionalFilters.get(currentPosition - 1);

				if (logger.isDebugEnabled()) {
					logger.debug(UrlUtils.buildRequestUrl(firewalledRequest)
							+ " at position " + currentPosition + " of " + size
							+ " in additional filter chain; firing Filter: '"
							+ nextFilter.getClass().getSimpleName() + "'");
				}

				nextFilter.doFilter(request, response, this);
			}
		}
	}

filter集合执行的逻辑在VirtualFilterChain的doFilter方法中。

filter是如何执行的

上面说了怎么才能进入filter的执行逻辑,下面说一下filter到底怎么执行,为什么一个
在VirtualFilterChain的doFilter方法可以执行所有的filter。

下面写一个例子,模拟filter的执行逻辑。
定义FilterChain接口、Filter接口:

public interface Filter {

    void doFilter(String username, int age, FilterChain filterChain);
}
public interface FilterChain {

    void doFilter(String username, int age);
}

定义两个Filter实现:

public class NameFilter implements Filter {

    @Override
    public void doFilter(String username, int age, FilterChain filterChain) {

        username = username + 1;
        System.out.println("username: " + username + "   age: " + age);
        System.out.println("正在执行:NameFilter");
        filterChain.doFilter(username, age);
    }
}
public class AgeFilter implements Filter {

    @Override
    public void doFilter(String username, int age, FilterChain filterChain) {

        age += 10;
        System.out.println("username: " + username + "   age: " + age);
        System.out.println("正在执行:AgeFilter");
        filterChain.doFilter(username, age);
    }
}

定义一个FilterChain实现:

public class FilterChainProxy implements FilterChain {


    private int position = 0;
    private int size = 0;
    private List<Filter> filterList = new ArrayList<>();

    public void addFilter(Filter filter) {

        filterList.add(filter);
        size++;
    }

    @Override
    public void doFilter(String username, int age) {

        if (size == position) {
            System.out.println("过滤器链执行结束");
        } else {

            Filter filter = filterList.get(position);
            position++;
            filter.doFilter(username, age, this);
        }
    }
}

测试Filter实现:

public class FilterTest {

    public static void main(String[] args) {

        FilterChainProxy proxy = new FilterChainProxy();
        proxy.addFilter(new NameFilter());
        proxy.addFilter(new AgeFilter());

        proxy.doFilter("张三", 0);
    }
}
=======
username: 张三1   age: 0
正在执行:NameFilter
username: 张三1   age: 10
正在执行:AgeFilter
过滤器链执行结束

在这个执行逻辑中,最重要的是【this】,this就是初始化的好的FilterChain实例,在这个测试实例中,this就是FilterChainProxy。

执行FilterChainProxy的doFilter方法的时候,传入了初始参数username和age,进入这个方法后,根据position取出相应的Filter,初次进入position是0,执行Filter的doFilter方法,注意,此时Filter的doFilter方法额外传入了一个this参数,这个参数就是初始化的好的FilterChain实例,在Filter中的doFilter的方法中最后又会执行FilterChain的doFilter方法,相当于第二次调用FilterChain实例的doFilter方法,此时posotion是1,然后再执行Filter的doFilter方法,直到所有的Filter执行完,整个执行过程结束。

VirtualFilterChain的doFilter方法的执行逻辑和这个测试实例中的执行逻辑基本一致。
这样就完成了整个过滤器链的执行。

总结

以前用Filter的时候就非常疑惑过滤器怎么执行的,直到今天才算解决了这个疑惑。

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