用于打印内容的JSP帮助程序类 [英] JSP helper class for printing content
问题描述
我对JSP中的代码重用有疑问。我有一个JSP页面 example.jsp ，它发出对数据库的调用并获得结果。我有一个java类 HelperClass.java 接受记录并打印出不同的字段
I have a question on code reuse in JSP. I have a JSP page example.jsp that issues a call to a database and gets the results. I have a java class HelperClass.java that accepts a record and prints out the different fields
response.getWriter().println
现在我的JSP页面也有HTML和问题是HelperClass打印出的内容出现在JSP页面中的内容之前。例如，
Now my JSP page has HTML as well and the problem is the content printed out by the HelperClass appears before the content in the JSP page. E.g.
<body>
This is the first line <br/>
HelperClass.printdata("second line");
</body>
输出
secondline This is the first line
这是一个已知问题。为将页面打印到页面的JSP页面设计 HelperClass 的最佳方法是什么？任何指针都将非常感激。
Is this a known issue. What is the best way to design an HelperClass for a JSP page that prints content out to the page. Any pointers would be greatly appreciated.
推荐答案
只是不要使用HelperClass来打印数据。这毫无意义。那里有EL for。
Just do not use a "HelperClass to print data". This makes no sense. There you have EL for.
${bean.property}
这就是全部。使用servlet来控制，预处理和后处理请求。使用taglibs（例如JSTL）和EL来访问和显示后端数据。
That's all. Use a servlet to control, preprocess and postprocess requests. Use taglibs (e.g. JSTL) and EL to access and display backend data.
这是一个Servlet的基本启动示例，它在JSP中显示之前预处理请求：
Here's a basic kickoff example of a Servlet which preprocesses the request before display in JSP:
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
List<Person> persons = personDAO.list(); // Get list of persons from DB.
request.setAttribute("persons", persons); // So it's available as `${persons}` in EL.
request.getRequestDispatcher("/WEB-INF/persons.jsp").forward(request, response); // Forward to JSP for display.
}
这里，人只是一个表示真实世界实体的Javabean类。
Here, the Person is just a Javabean class which represents a real world entity.
public class Person {
private Long id;
private String name;
private String email;
private Integer age;
// Add/generate getters and setters here.
}
PersonDAO＃list（）方法只返回数据库中 List 的 c> c>对象
The PersonDAO#list() method just returns a List of Person objects from the DB:
public List<Person> list() throws SQLException {
Connection connection = null;
Statement statement = null;
ResultSet resultSet = null;
List<Person> persons = new ArrayList<Person>();
try {
connection = database.getConnection();
statement = connection.createStatement("SELECT id, name, email, age FROM person");
resultSet = statement.executeQuery();
while (resultSet.next()) {
Person person = new Person();
person.setId(resultSet.getLong("id"));
person.setName(resultSet.getString("name"));
person.setEmail(resultSet.getString("email"));
person.setAge(resultSet.getInteger("age"));
persons.add(person);
}
} finally {
if (resultSet != null) try { resultSet.close(); } catch (SQLException logOrIgnore) {}
if (statement != null) try { statement.close(); } catch (SQLException logOrIgnore) {}
if (connection != null) try { connection.close(); } catch (SQLException logOrIgnore) {}
}
return persons;
}
将servlet映射到 web.xml 在 的 url-pattern 上。 JSP隐藏在 / WEB-INF 中，这样没有人可以直接访问它而不先请求servlet（否则会得到一个空表）。
Map the servlet in web.xml on an url-pattern of /persons. The JSP is hidden in /WEB-INF so that nobody can access it directly without requesting the servlet first (else one would get an empty table).
现在，这是 persons.jsp 的样子，它使用 JSTL （只需删除 / WEB-INF / lib 中的-1.2.jarrel =noreferrer> jstl-1.2.jar c：forEach 迭代 List ，它使用EL来访问后端数据和bean属性。 servlet将 List< Person> 作为请求属性，名称为 persons ，以便<$ c $ EL中的c> $ {persons} 。 c：forEach 中的每次迭代都会返回一个 Person 实例，以便您可以使用EL显示其优先级。
Now, here's how persons.jsp look like, it uses JSTL (just drop jstl-1.2.jar in /WEB-INF/lib) c:forEach to iterate over a List and it uses EL to access the backend data and bean properties. The servlet has put the List<Person> as request attribute with name persons so that it's available by ${persons} in EL. Each iteration in c:forEach gives a Person instance back, so that you can display their proeprties with EL.
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
...
<table>
<c:forEach items="${persons}" var="person">
<tr>
<td>${person.name}</td>
<td>${person.email}</td>
<td>${person.age}</td>
</tr>
</c:forEach>
</table>
通过 http://example.com/contextname/persons 。就这样。无需HelperClass打印数据;）要了解有关JSTL的更多信息，请查看 Java EE教程第二部分第7章并了解有关EL的更多信息，请查看 Java EE教程第二部分第5章。要了解有关PersonDAO背后的更多信息，请查看这篇文章 。
Call it by http://example.com/contextname/persons. That's all. No need for a "HelperClass to print data" ;) To learn more about JSTL, check Java EE tutorial part II chapter 7 and to learn more about EL, check Java EE tutorial part II chapter 5. To learn more about stuff behind PersonDAO, check this article.
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