代码需要JSP声明部分中的try / catch，但不需要在scriptlet部分中吗？ [英] Code requires try/catch in JSP declaration section but not in the scriptlet section?

问题描述

我有以下代码块可以正常工作：

I have the following block of code that works just fine:

<%@page import="java.util.*" %>
<%@page import="java.security.*" %>

<%
String str = "A string to hash.";
MessageDigest md = MessageDigest.getInstance("MD5");
md.update( str.getBytes() );
byte[] digest = md.digest();
StringBuffer hexString = new StringBuffer();
for (int i = 0, j = digest.length; i < j; i++) {
    String tmp = Integer.toHexString(0xFF & digest[i]);
    if (tmp.length() < 2) {
        tmp = "0" + tmp;
    }

    hexString.append(tmp);
}

out.println(hexString.toString());
%>

当我试图将散列代码分解为方法时，我得到了NoSuchAlgorithmException定义MessageDigest对象时出错：

When I tried to break the hashing code out into a method I got a "NoSuchAlgorithmException" error when defining the MessageDigest object:

<%@page import="java.util.*" %>
<%@page import="java.security.*" %>

<%
String str = "A string to hash";
String md5string = md5hash(str);

out.println(md5string);
%>

<%!
public String md5hash(String str) {
    MessageDigest md = MessageDigest.getInstance("MD5");

    md.update( str.getBytes() );
    byte[] digest = md.digest();
    StringBuffer hexString = new StringBuffer();
    for (int i = 0, j = digest.length; i < j; i++) {
        String tmp = Integer.toHexString(0xFF & digest[i]);
        if (tmp.length() < 2) {
            tmp = "0" + tmp;
        }

        hexString.append(tmp);
    }

    return hexString.toString();
}
%>

为了让JSP编译，我不得不修改它所以：

To get the JSP to compile, I had to modify it like so:

<%@page import="java.util.*" %>
<%@page import="java.security.*" %>

<%
String str = "A string to hash";
String md5string = md5hash(str);

out.println(md5string);
%>

<%!
public String md5hash(String str) {
    MessageDigest md = null;

    try {
        md = MessageDigest.getInstance("MD5");
    } catch (NoSuchAlgorithmException e) {}

    md.update( str.getBytes() );
    byte[] digest = md.digest();
    StringBuffer hexString = new StringBuffer();
    for (int i = 0, j = digest.length; i < j; i++) {
        String tmp = Integer.toHexString(0xFF & digest[i]);
        if (tmp.length() < 2) {
            tmp = "0" + tmp;
        }

        hexString.append(tmp);
    }

    return hexString.toString();
}
%>

为什么我要添加一个无用的try / catch来制作这段代码有用吗？

Why did I have to add a useless try/catch to make this code work?

推荐答案

普通的JSP源代码本身已经放在一个巨大的try-catch块中。每个普通的scriptlet < %%> 都将成为其中的一部分，因此您无需显式捕获异常。但是，方法定义<％！ ％> 将置于标准的try-catch之外，因此您必须自己处理这些例外。

The normal JSP source code is by itself already placed in one huge try-catch block. Every normal scriptlet <% %> will become part of it, so you don't need to explicitly catch the exceptions. However, a method definition <%! %> will be placed outside the standard try-catch, so you have to handle the exceptions yourself.

不用说，这不是最好的做法。而是将Java代码放在真正的Java类中。出于这个特殊目的，我认为EL函数非常有用。另请参见如何避免JSP文件中的Java代码？

Needless to say that this is not the best practice. Rather put the Java code in a real Java class. For this particular purpose, I think an EL function is very useful. See also How to avoid Java code in JSP files?

这篇关于代码需要JSP声明部分中的try / catch，但不需要在scriptlet部分中吗？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！