JSF 捕獲 Managed Bean 建構子異常：VariableResolver 切入點實戰

問題背景

Action 方法拋出異常可用 ActionListener 捕獲，但 Managed Bean 建構子異常 發生在建構階段，Filter 僅能捕獲包裝後的 ServletException，無法還原原始異常。

解決方案：自訂 VariableResolver

JSF 透過 VariableResolver 解析並實例化 Managed Bean。繼承實作類並覆寫 resolveVariable 即可攔截建構異常。

MyFaces 環境範例

public class VariableResolverImpl extends org.apache.myfaces.el.VariableResolverImpl {
    private static final String ERROR_FLAG = "error";
    private static final String EXCEPTION_TYPE = "exception";

    @Override
    public Object resolveVariable(FacesContext facesContext, String name) {
        try {
            return super.resolveVariable(facesContext, name);
        } catch (Throwable e) {
            HttpServletRequest request = (HttpServletRequest) 
                facesContext.getExternalContext().getRequest();
            request.setAttribute(ERROR_FLAG, Boolean.TRUE);
            request.setAttribute(EXCEPTION_TYPE, e);
            throw new RuntimeException(e); // 保持原始堆疊
        }
    }
}

註冊至 faces-config.xml

<faces-config>
  <application>
    <variable-resolver>mypackage.VariableResolverImpl</variable-resolver>
  </application>
</faces-config>

Filter 中處理原始異常

public class AppFilter implements Filter {
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) 
            throws IOException, ServletException {
        try {
            chain.doFilter(req, resp);
        } catch (ServletException e) {
            Boolean error = (Boolean) req.getAttribute("error");
            if (Boolean.TRUE.equals(error)) {
                Throwable te = (Throwable) req.getAttribute("exception");
                if (te instanceof AppException) {
                    // 業務異常處理
                } else {
                    // 系統異常處理
                }
            } else {
                // 其他 ServletException
            }
        }
    }
}

關鍵點總結

• 切入點：VariableResolver.resolveVariable() —— Bean 實例化瞬間
• 異常透傳：Request 屬性攜帶原始 Throwable
• Filter 最後統一處理，避免業務代碼散落
• 適用於安全檢查、資源初始化失敗、配置錯誤等建構期異常