From JSP 1.2 Specification, it is highly recommended that the JSP Standard Tag Library (JSTL) be used in your web application to help reduce the need for JSP scriptlets in your pages. Pages that use JSTL are, in general, easier to read and maintain.
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Where possible, avoid JSP scriptlets whenever tag libraries provide equivalent functionality. This makes pages easier to read and maintain, helps to separate business logic from presentation logic, and will make your pages easier to evolve into JSP 2.0-style pages (JSP 2.0 Specification supports but de-emphasizes the use of scriptlets).
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In the spirit of adopting the model-view-controller (MVC) design pattern to reduce coupling between the presentation tier from the business logic, JSP scriptlets should not be used for writing business logic. Rather, JSP scriptlets are used if necessary to transform data (also called "value objects") returned from processing the client's requests into a proper client-ready format. Even then, this would be better done with a front controller servlet or a custom tag.
If you want to invoke the same Java code on every request, less-or-more regardless of the requested page, e.g. checking if a user is logged in, then implement a filter and write code accordingly in doFilter()
method. E.g.:
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws ServletException, IOException {
if (((HttpServletRequest) request).getSession().getAttribute("user") == null) {
((HttpServletResponse) response).sendRedirect("login"); // Not logged in, redirect to login page.
} else {
chain.doFilter(request, response); // Logged in, just continue request.
}
}
When mapped on an appropriate <url-pattern>
covering the JSP pages of interest, then you don't need to copypaste the same piece of code overall JSP pages.
If you want to invoke some Java code to process a GET request, e.g. preloading some list from a database to display in some table, if necessary based on some query parameters, then implement a servlet and write code accordingly in doGet()
method. E.g.:
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
try {
List<Product> products = productService.list(); // Obtain all products.
request.setAttribute("products", products); // Store products in request scope.
request.getRequestDispatcher("/WEB-INF/products.jsp").forward(request, response); // Forward to JSP page to display them in a HTML table.
} catch (SQLException e) {
throw new ServletException("Retrieving products failed!", e);
}
}
This way dealing with exceptions is easier. The DB is not accessed in the midst of JSP rendering, but far before the JSP is been displayed. You still have the possibility to change the response whenever the DB access throws an exception. In the above example, the default error 500 page will be displayed which you can anyway customize by an <error-page>
in web.xml
.
If you want to invoke some Java code to process a POST request, such as gathering data from a submitted HTML form and doing some business stuff with it (conversion, validation, saving in DB, etcetera), then implement a servlet and write code accordingly in doPost()
method. E.g.:
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String username = request.getParameter("username");
String password = request.getParameter("password");
User user = userService.find(username, password);
if (user != null) {
request.getSession().setAttribute("user", user); // Login user.
response.sendRedirect("home"); // Redirect to home page.
} else {
request.setAttribute("message", "Unknown username/password. Please retry."); // Store error message in request scope.
request.getRequestDispatcher("/WEB-INF/login.jsp").forward(request, response); // Forward to JSP page to redisplay login form with error.
}
}
This way dealing with different result page destinations is easier: redisplaying the form with validation errors in case of an error (in this particular example you can redisplay it using ${message}
in EL), or just taking to the desired target page in case of success.
If you want to invoke some Java code to control the execution plan and/or the destination of the request and the response, then implement a servlet according to the MVC's Front Controller Pattern. E.g.:
protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
try {
Action action = ActionFactory.getAction(request);
String view = action.execute(request, response);
if (view.equals(request.getPathInfo().substring(1)) {
request.getRequestDispatcher("/WEB-INF/" + view + ".jsp").forward(request, response);
} else {
response.sendRedirect(view);
}
} catch (Exception e) {
throw new ServletException("Executing action failed.", e);
}
}
Or just adopt an MVC framework like JSF, Spring MVC, Wicket, etc so that you end up with just a JSP/Facelets page and a JavaBean class without the need for a custom servlet.
If you want to invoke some Java code to control the flow inside a JSP page, then you need to grab an (existing) flow control taglib like JSTL core. E.g. displaying List<Product>
in a table:
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
...
<table>
<c:forEach items="${products}" var="product">
<tr>
<td>${product.name}</td>
<td>${product.description}</td>
<td>${product.price}</td>
</tr>
</c:forEach>
</table>
With XML-style tags which fit nicely among all that HTML, the code is better readable (and thus better maintainable) than a bunch of scriptlets with various opening and closing braces ("Where the heck does this closing brace belong to?"). An easy aid is to configure your web application to throw an exception whenever scriptlets are still been used by adding the following piece to web.xml
:
<jsp-config>
<jsp-property-group>
<url-pattern>*.jsp</url-pattern>
<scripting-invalid>true</scripting-invalid>
</jsp-property-group>
</jsp-config>
In Facelets, the successor of JSP, which is part of the Java EE provided MVC framework JSF, it is already not possible to use scriptlets. This way you're automatically forced to do things "the right way".
If you want to invoke some Java code to access and display "backend" data inside a JSP page, then you need to use EL (Expression Language), those ${}
things. E.g. redisplaying submitted input values:
<input type="text" name="foo" value="${param.foo}" />
The ${param.foo}
displays the outcome of request.getParameter("foo")
.
If you want to invoke some utility Java code directly in the JSP page (typically public static
methods), then you need to define them as EL functions. There's a standard functions taglib in JSTL, but you can also easily create functions yourself. Here's an example how JSTL fn:escapeXml
is useful to prevent XSS attacks.
<%@ taglib uri="http://java.sun.com/jsp/jstl/functions" prefix="fn" %>
...
<input type="text" name="foo" value="${fn:escapeXml(param.foo)}" />
Note that the XSS sensitivity is in no way specifically related to Java/JSP/JSTL/EL/whatever, this problem needs to be taken into account in every web application you develop. The problem of scriptlets is that it provides no way of builtin preventions, at least not using the standard Java API. JSP's successor Facelets has already implicit HTML escaping, so you don't need to worry about XSS holes in Facelets.
Best Answer
First off, some clarifications to KLE's answer:
Unconstrained (nullable) one-to-one association is the only one that can not be proxied without bytecode instrumentation. The reason for this is that owner entity MUST know whether association property should contain a proxy object or NULL and it can't determine that by looking at its base table's columns due to one-to-one normally being mapped via shared PK, so it has to be eagerly fetched anyway making proxy pointless. Here's a more detailed explanation.
many-to-one associations (and one-to-many, obviously) do not suffer from this issue. Owner entity can easily check its own FK (and in case of one-to-many, empty collection proxy is created initially and populated on demand), so the association can be lazy.
Replacing one-to-one with one-to-many is pretty much never a good idea. You can replace it with unique many-to-one but there are other (possibly better) options.
Rob H. has a valid point, however you may not be able to implement it depending on your model (e.g. if your one-to-one association is nullable).
Now, as far as original question goes:
A)
@ManyToOne(fetch=FetchType.LAZY)
should work just fine. Are you sure it's not being overwritten in the query itself? It's possible to specifyjoin fetch
in HQL and / or explicitly set fetch mode via Criteria API which would take precedence over class annotation. If that's not the case and you're still having problems, please post your classes, query and resulting SQL for more to-the-point conversation.B)
@OneToOne
is trickier. If it's definitely not nullable, go with Rob H.'s suggestion and specify it as such:Otherwise, if you can change your database (add a foreign key column to owner table), do so and map it as "joined":
and in OtherEntity:
If you can't do that (and can't live with eager fetching) bytecode instrumentation is your only option. I have to agree with CPerkins, however - if you have 80!!! joins due to eager OneToOne associations, you've got bigger problems then this :-)