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THE CHALLENGE OF DIFFERENT APPLICATION SERVERS
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in a browser to diagnose problems, or use HttpUnit tests to fetch the values, parse them, and verify that the server is what you expected. We make do with a JSP to list the properties, neglecting security manager issues until we encounter them. We should password protect this file and all other administrative pages, of course.
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<%@ page language="java" %> <%@ page session="false" %> <%@ page import="java.util.Enumeration" %> <%@ page import="java.util.Properties" %> <html><body><table> <% Properties props=System.getProperties(); for (Enumeration e = props.propertyNames(); e.hasMoreElements();) { String key=(String)e.nextElement(); String value=props.getProperty(key); %> <tr><td><%=key%></td><td><%=value%></td></tr> <% } %> </table></body></html>
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This page is purely for human diagnostics; we do not run any automated tests against it. We also want to tack in some version tests. Asserting which libraries must be included in the system Since section 15.4.4, our <happy> JSP tag has offered a way of asserting that a named class must exist. This lets you dictate the minimum version of Java supported, such as with a statement that Java 1.4 or later is required:
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<happy:happy classMustExist="java.lang.CharSequence" errorText="We need Java 1.4 or later"/>
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This technique is an effective way of setting an absolute barrier against old versions of Java. This assertion technique is useful when you have many external libraries that you depend upon; a single JSP can probe for all of them, providing a single file you can retrieve to verify that the libraries are all present and available. You could fetch this file by hand; we point you at <get>, HttpUnit, and Canoo WebTest as the automated choices. 18.1.3 Coping with different API implementations A server may give you its own implementations of the JAXP APIs, or other elements of the J2EE library, or other standard packages such as the Java management API, JMX. If you redistribute your own versions things may break; if you rely on the supplied version you need to retest everything. As an example, many complex applications depend upon Xerces being present; if your application server supplies something different, things might break.
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CHAPTER 1 8
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Usually experimentation and experience help you determine what works and what doesn t. We went back to our HappyTag.java tag for configuration and added a test for the XML parser, one that extracts the parser name and verifies that it found the string we passed as parser name. See listing 18.1.
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Listing 18.1 Additions to HappyTag.java to verify parsers
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private String parserName = null; /** *@jsp:attribute required="false" */ public void setParserName(String parserName) { this.parserName = parserName; } public void testParserName() throws JspException { if(parserName!=null) { String parser=getParserName(); if(parser.indexOf(parserName) == -1) { throw new JspException("Parser "+parserName +" was not found; we are using " +parser); } } } public String getParserName() throws JspException { try { SAXParserFactory saxParserFactory = SAXParserFactory.newInstance(); SAXParser saxParser = saxParserFactory.newSAXParser(); String saxParserName = saxParser.getClass().getName(); return saxParserName; } catch (Exception e) { throw new JspException(e); } }
With the addition of a call to testParserName() in our class s doStartTag() method, we can then have a test in a file such as
<happy:happy parserName="crimson"/>
When this page is loaded, either everything is happy, or we see an error such as
javax.servlet.jsp.JspException: Parser crimson was not found; we are using org.apache.xerces.jaxp.SAXParserImpl
Throwing an error in this situation is a bit extreme as almost everything that works with Crimson should work with Xerces. However, the opposite is not true: there are
THE CHALLENGE OF DIFFERENT APPLICATION SERVERS
many things that work with Xerces, but not Crimson. We can enforce our need for Xerces by stating this in the happiness test:
<happy:happy parserName="xerces"/>
There is no point running this test every 15 minutes on a live server, but when you bring up a new system or run regression tests after an update, you should run through all these tests; the HttpUnit tests should fetch the happy.jsp health page after every deployment. These tests are somewhat brittle against improvements in the underlying system; we don t know that a SAX implementation bound via JNI to the Expat XML parser won t work; we just haven t tested it yet. However, we take the view that the choice of implementations of XML parsers, web service APIs, and the like is so important we need to make sure that the run time provides the versions we want. 18.1.4 Vendor-specific libraries Sometimes you need access to vendor-supplied libraries. For a cross-platform application, you need to call vendor-specific classes using reflection, or simply by having wrapper classes that provide access; your application must then load the appropriate wrapper class at run time. Ant provides no help in this process, unless you want selective inclusion of source files at compile time. What you can do with Ant is explicitly differentiate between those libraries you depend upon at build time, and those you include in the WAR file. Deployment descriptors The deployment descriptors for each platform often need tuning. We showed how to generate custom web.xml files in section 12.4. The mechanics of customizing deployment descriptors are relatively straightforward with XDoclet, though determining what needs tuning is hard. Targeting different application servers with EJBs is complicated, because you need to generate the server-specific deployment descriptors. Here you can use the different <ejbdoclet> subtasks, and the different <ejbjar> nested tasks to create the specific EJB JAR files for your target system. Server-specific deployment processes The actual deployment mechanism for each platform can vary wildly. Ant can address this, and you can always deploy by hand until you are ready to write the Ant support. Although this can be labor intensive, it is actually one of the least dangerous differences between servers, and much less worrisome than subtle run-time variations. Server-specific management The operational aspects of the server security, performance, load balancing options, and the management interface are usually significantly different. There is little Ant can do in this area. When the Java Community Process working group on server
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