Java Bean:如何在<jsp:useBean>中生成<jsp:setProperty>到Java代码中

Question

例如,我有这个代码:

<jsp:useBean id="dog" class="Dog" scope="application">
     <jsp:setProperty name="dog" property="breed" value="House Dog !!!"/>
</jsp:useBean>

我知道它是如何工作的.但是,有时候,我在这里更改了一些代码,例如:"dog"到"newDog",我会遇到错误或未经编辑的结果(和我一起).

请告诉我如何将上面的代码生成到Java中.(也许只是一个主要想法)

谢谢 :)

Answer 1

JSP最终生成到.java被编译为servlet的类.检查服务器的工作文件夹.在Tomcat的情况下,在Tomcat的文件夹中/test.jsp生成一个文件./org/apache/jsp/test_jsp.java/work

以下几行

<jsp:useBean id="dog" class="com.example.Dog" scope="application">
     <jsp:setProperty name="dog" property="breed" value="House Dog !!!"/>
</jsp:useBean>

(我唯一的改变就是添加一个包;无包装类是Bad™)

生成为

  com.example.Dog dog = null;
  synchronized (application) {
    dog = (com.example.Dog) _jspx_page_context.getAttribute("dog", javax.servlet.jsp.PageContext.APPLICATION_SCOPE);
    if (dog == null){
      dog = new com.example.Dog();
      _jspx_page_context.setAttribute("dog", dog, javax.servlet.jsp.PageContext.APPLICATION_SCOPE);
      out.write("\n");
      out.write("     ");
      org.apache.jasper.runtime.JspRuntimeLibrary.introspecthelper(_jspx_page_context.findAttribute("dog"), "breed", "House Dog !!!", null, null, false);
      out.write('\n');
    }
  }

Tomcat是开源的,根据其源代码,该JspRuntimeLibrary#introspecthelper()方法委托internalIntrospecthelper()最终执行此操作:

Method method = null;
Class<?> type = null;
Class<?> propertyEditorClass = null;
try {
    java.beans.BeanInfo info
        = java.beans.Introspector.getBeanInfo(bean.getClass());
    if ( info != null ) {
        java.beans.PropertyDescriptor pd[]
            = info.getPropertyDescriptors();
        for (int i = 0 ; i < pd.length ; i++) {
            if ( pd[i].getName().equals(prop) ) {
                method = pd[i].getWriteMethod();
                type   = pd[i].getPropertyType();
                propertyEditorClass = pd[i].getPropertyEditorClass();
                break;
            }
        }
    }
    if ( method != null ) {
        if (type.isArray()) {
            if (request == null) {
                throw new JasperException(
                    Localizer.getMessage("jsp.error.beans.setproperty.noindexset"));
            }
            Class<?> t = type.getComponentType();
            String[] values = request.getParameterValues(param);
            //XXX Please check.
            if(values == null) return;
            if(t.equals(String.class)) {
                method.invoke(bean, new Object[] { values });
            } else {
                createTypedArray (prop, bean, method, values, t,
                                  propertyEditorClass); 
            }
        } else {
            if(value == null || (param != null && value.equals(""))) return;
            Object oval = convert(prop, value, type, propertyEditorClass);
            if ( oval != null )
                method.invoke(bean, new Object[] { oval });
        }
    }
} catch (Exception ex) {
    Throwable thr = ExceptionUtils.unwrapInvocationTargetException(ex);
    ExceptionUtils.handleThrowable(thr);
    throw new JasperException(ex);
}

你看,它java.beans.Introspector用于获取bean信息和属性BeanInfo#getPropertyDescriptors().所需的<jsp:setProperty>方法如获得java.lang.reflect.Method通过PropertyDescriptor#getWriteMethod().最后,它使用Reflection API来调用该方法.