Using the SCA JSP Reference Tag in Java

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Listing 124 Using the SCA JSP Reference Tag
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<%@ page contentType="text/html;charset=UTF-8" language="java" %> <!-- body of the JSP --> <sca:reference name="loanService" type="bigbankLoanService"/> <!-- --> <% LoanApplication application = loanServiceapply(application) %>
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In the preceding JSP fragment, the <sca:reference> tag declares the loanService reference, with a service contract as de ned by the bigbankLoanService Java interface Using the tag has two effects First, it declares a reference for the web component that is wired in the webcomposite le Assuming this JSP and the servlet from the earlier example both declared the loanService reference, the webcomposite le would remain the same as in Listing 123 In other words, the web component de nition would contain only one <reference> entry When the web component is deployed to the domain, the SCA runtime will ensure that reference proxies are available to all servlets and JSPs that declare it The second effect of the JSP <sca:reference> tag is that it makes the reference proxy available in the JSP page context using loanService as the variable name As seen in Listing 124, the reference proxy can be invoked using inline Java Generally, using inline Java in JSPs is considered bad practice as it mixes rendering logic (HTML and JavaScript) with application logic An example that would align more closely with JSP best practices would use other tags to access the reference proxy Because the reference proxy is made available in the JSP page context, it can be accessed using JSP expressions For example, assume BigBank has another JSP that displays current rates using RateService The current rates returned from the service can be iterated and displayed using built-in JSP tags and the JSP expression language, as shown in Listing 125
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Listing 125 Accessing a Reference Proxy Using JSP Tags
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<%@ page contentType="text/html;charset=UTF-8" language="java" %> <%@ taglib pre x="c" uri="http://javasuncom/jsp/jstl/core" %> <%@ taglib pre x="sca" uri="http://wwwosoaorg/sca/scatld" %> <sca:reference name="rateService" type="bigbankRateService"/> <html> <body> <table> <c:forEach items="${rateServicerates}" var="rate"> <tr> <td${ratetype} at ${ratepercentage}% and ${rateapr} APR</td> </tr>
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</c:forEach> </body> </html>
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To use the reference tag, you need to include the tag library jar in the web component WAR under the WEB-INF/lib directory SCA runtimes that support web components (for example, Fabric3) make this tag library available as part of the runtime distribution or development kit After you have obtained the tag library, it must be declared in a JSP using the taglib directive, as shown in Listing 126
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Listing 126 Using the Taglib Directive with the SCA Tag Library
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<%@ page contentType="text/html;charset=UTF-8" language="java" %> <%@ taglib pre x="sca" uri="http://wwwosoaorg/sca/scatld" %> <!--jsp contents -->
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At times, it is useful to avoid blocking on a request before returning a response to a browser client In these situations, servlets can use non-blocking operations on SCA services Assuming the LoanServiceapply() operation is marked with the @OneWay annotation and is long-running, the servlet in the example shown in Listing 127 will return a response to the client before the service provider has completed processing
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Listing 127 A Servlet Invoking a Non-Blocking Operation on an SCA Service
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public class BigBankLoanServlet extends HttpServlet{ @Reference protected LoanService loanService; protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { LoanApplication application = // loanServiceapply(application); // returns immediately as the call is non-blocking } }
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Asynchronous invocations can improve scalability because the runtime does not need to hold open client connections while processing is being done This is particularly the case when operations may require a signi cant amount of time to complete The main drawback to using asynchronous invocations in servlets is that error handling becomes more dif cult For example, if an error occurs while processing the loan, the user will not receive immediate feedback because the servlet will have already returned a response to the client Instead, error handling would need to be done by LoanComponent (that is, the component providing LoanService), with possibly a noti cation sent to the user via email or some other communications channel Although servlets and JSPs may invoke non-blocking operations, they cannot receive callbacks If a service is bidirectional (that is, it speci es a callback service), it must be accessed from an intermediary service that implements the required callback interface To understand how this works, let s return to the CreditService callback example we introduced in 3, Service-Based Development Using Java The CreditService and CreditServiceCallback interfaces are de ned in Listing 128
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