异常类的妙用

以往在使用异常时，只是知道通过异常类的构造方法设置一些出错信息,此外最多就是把引起该异常的原因通过Throwable类的子类一同设置进去。今天在分析springSecurity3.0.5的框架时，看到AuthenticationException这个异常类(用在验证时)的源码，发现这个异常类除了上述常见的用法外，还把Authentication（验证信息）的做为属性放到了异常类中，当验证出错时AuthenticationException抛出时，Authentication（验证信息）一同被抛出了，这样捕获异常的类就能得到更丰富的信息，有些数据还可以通过异常类返回给调用者。看了这样的代码，思路不由地开阔了不少。该类源码如下：

package org.springframework.security.core;

/**
 * Abstract superclass for all exceptions related to an {@link Authentication} object being invalid for whatever
 * reason.
 *
 * @author Ben Alex
 */
public abstract class AuthenticationException extends RuntimeException {
    //~ Instance fields ================================================================================================

    private Authentication authentication;
    private Object extraInformation;

    //~ Constructors ===================================================================================================

    /**
     * Constructs an <code>AuthenticationException</code> with the specified message and root cause.
     *
     * @param msg the detail message
     * @param t the root cause
*/
    public AuthenticationException(String msg, Throwable t) {
        super(msg, t);
    }

    /**
     * Constructs an <code>AuthenticationException</code> with the specified message and no root cause.
     *
     * @param msg the detail message
*/
    public AuthenticationException(String msg) {
        super(msg);
    }

    public AuthenticationException(String msg, Object extraInformation) {
        super(msg);
        this.extraInformation = extraInformation;
    }

    //~ Methods ========================================================================================================

    /**
     * The authentication request which this exception corresponds to (may be <code>null</code>)
*/
    public Authentication getAuthentication() {
        return authentication;
    }

    public void setAuthentication(Authentication authentication) {
        this.authentication = authentication;
    }

    /**
     * Any additional information about the exception. Generally a <code>UserDetails</code> object.
     *
     * @return extra information or <code>null</code>
*/
    public Object getExtraInformation() {
        return extraInformation;
    }

    public void clearExtraInformation() {
        this.extraInformation = null;
    }
}

下面是在AbstractAuthenticationManager类中被使用到的片断：

 public final Authentication authenticate(Authentication authRequest) throws AuthenticationException {
        try {
            return doAuthentication(authRequest);
        } catch (AuthenticationException e) {
            e.setAuthentication(authRequest);

            if (clearExtraInformation) {
                e.clearExtraInformation();
            }

            throw e;
        }
    }

 通过上述两段源码可以看到，老外在写代码时考虑得很细，在异常类中加入了额外信息后,还提供了一个 clearExtraInformation()方法，用来清除额外信息。