When the Java code throws an exception, Resin will immediately stop the script and report the error to the browser. For example, we'll force a null pointer exception:

<%@ page language=java %>

<% String foo = null; %>
<%= foo.length() %>

The reported error looks something like the following:

500 Servlet Exception java.lang.NullPointerException at work.test.buggy$_jsp._jspService(/test/buggy.jsp:4) at com.caucho.jsp.JavaPage.service(JavaPage.java:72) at com.caucho.jsp.QServlet.service(QServlet.java:152) at com.caucho.server.http.Invocation.service(Invocation.java:183) at com.caucho.server.http.Request.dispatch(Request.java:174) at com.caucho.server.http.HttpRequest.handleRequest(HttpRequest.java:151) at com.caucho.server.http.HttpRequest.handleConnection(HttpRequest.java:114) at com.caucho.server.TcpConnection.run(TcpConnection.java:115) at java.lang.Thread.run(Thread.java)

The most important part of the error message is the location: `/test/buggy.jsp:4'. Resin translates the line in the generated Java code back to the JSP line.

What if I get 'Compiled Code' instead of lines?

You should run Resin with -nojit, turning off the just-in-time compiler, when developing so you can see the proper line numbers. That alone will make development much easier.

Java VMs with just-in-time compilers (jits) compile Java bytecodes to machine code. This makes Java almost as fast as C, but removes all the line numbers. Running with -nojit will disable the compiler. When your code is completely debugged, you can run with the compiler enabled.

To see the difference, here's the same stack trace with a JIT enabled:

500 Servlet Exception java.lang.NullPointerException at work.test.buggy$_jsp._jspService(Compiled Code) at com.caucho.jsp.JavaPage.service(Compiled Code) at com.caucho.jsp.QServlet.service(Compiled Code) at com.caucho.server.http.Invocation.service(Compiled Code) at com.caucho.server.http.Request.dispatch(Compiled Code) at com.caucho.server.http.HttpRequest.handleRequest(Compiled Code) at com.caucho.server.http.HttpRequest.handleConnection(Compiled Code) at com.caucho.server.TcpConnection.run(Compiled Code) at java.lang.Thread.run(Compiled Code)

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