Two common problems in compiling Servlets for use within a Tomcat container ...

---

PROBLEM: My Java Compile of a Servlet says "javax.servlet cannot be resolved"

For example:

[trainee@easterton sources]$ javac Smallest.java
----------
1. ERROR in Smallest.java
 (at line 2)
   import javax.servlet.*;
          ^^^^^^^^^^^^^
The import javax.servlet cannot be resolved
----------
2. ERROR in Smallest.java
 (at line 3)
   import javax.servlet.http.*;
          ^^^^^^^^^^^^^
The import javax.servlet cannot be resolved
----------
3. ERROR in Smallest.java
 (at line 5)
   public class Smallest extends HttpServlet {
                                 ^^^^^^^^^^^
HttpServlet cannot be resolved to a type
----------
4. ERROR in Smallest.java
 (at line 9)
   public void doGet(HttpServletRequest request,
                     ^^^^^^^^^^^^^^^^^^
HttpServletRequest cannot be resolved to a type
----------
5. ERROR in Smallest.java
 (at line 10)
   HttpServletResponse response)
   ^^^^^^^^^^^^^^^^^^^
HttpServletResponse cannot be resolved to a type
----------
6. ERROR in Smallest.java
 (at line 11)
   throws IOException, ServletException
                       ^^^^^^^^^^^^^^^^
ServletException cannot be resolved to a type
----------
6 problems (6 errors)[trainee@easterton sources]$


SOLUTION: Add the Servlet API to your CLASSPATH.

[trainee@easterton sources]$ export CLASSPATH=/usr/local/tomcat/common/lib/servlet-api.jar 
[trainee@easterton sources]$ javac Smallest.java
[trainee@easterton sources]$

EXPLANATION:

Even if you're just compiling a Servlet to produce a class file, on a machine on which you won't actually be running it, the Java compiler insists on having the necessary classes available to check you code / to know what it is extending.

The Servelet API jar file is available from within a standard Tomcat download, and also from within a Java Enterprise download, but it is not downloaded with the standard edition!

---

PROBLEM: I get an error message about "does not declare serialVersionUID"

[trainee@easterton sources]$ javac Smallest.java----------
1. WARNING in Smallest.java
 (at line 5)
   public class Smallest extends HttpServlet {
                ^^^^^^^^
The serializable class Smallest does not declare a static final serialVersionUID field of type long
----------
[trainee@easterton sources]$

SOLUTION: Check that you're using the correct Java compiler!

1 problem (1 warning)[trainee@easterton sources]$ which java
/usr/bin/java
[trainee@easterton sources]$ export PATH=/usr/local/java/bin:$PATH
[trainee@easterton sources]$ javac Smallest.java
[trainee@easterton sources]$

EXPLANATION: There are various Java Compilers available, and the one that's shipped with Linux releases is highly unlikely bo be Sun's one which you'll want to use with Servlets / Tomcat's classes.

If you have Tomcat downloaded and installed already, chances are that you do have the right compiler already on your system - if not, download it from Sun - but that your PATH to executable files is currently pointing at the one shipped with the operating system.

---

Learn all about these on our Deploying Apache httpd and Tomcat course!
(written 2008-11-26)

 J906 - Java - Servlets in More Detail
  [479] New servlet from old - (2005-10-28)
  [1495] Single login and single threaded models - Java and PHP - (2008-01-04)
  [1550] Java (JSP and Servlet examples) live on our server - (2008-02-23)
  [2183] Servlet life cycle, and Java Servlet variable scope - (2009-05-16)
  [2652] Reading and writing cookies in Java Servlets and JSPs - (2010-02-26)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [3044] Changing a Servlet - more that just editing and compiling - (2010-11-12)
  [3293] Distributing the server load - yet ensuring that each user return to the same system (Apache httpd and Tomcat) - (2011-05-18)
  [4431] A Java servlet that is also a stand alone program. And a server that is also a web client. - (2015-02-19)
  [4432] Java web application for teaching - now with sessions and clustering / load balancing demonstrations - (2015-02-20)

J901 - Java - Servlets
  [3997] Servlet v JSP (Java Server Page). What is the difference? - (2013-02-06)

A653 - Web Application Deployment - Tomcat - Configuring Web Applications
  [465] Changing Tomcat's web.xml and reloading a web application - (2005-10-15)
  [754] tar, jar, war, ear, sar files - (2006-06-10)
  [1006] Apache httpd and Apache Tomcat together tips - (2006-12-24)
  [1074] Java beans and classes, .war and .jar files - (2007-02-10)
  [1082] Straight from the .jar - (2007-02-16)
  [1192] What are WEB-INF and META-INF directories? - (2007-05-18)
  [1552] Extra public classes in deploying Apache httpd and Tomcat - (2008-02-24)
  [1564] Default file (MiMe types) for Apache httpd and Apache Tomcat - (2008-03-04)
  [2088] Changing the 404 - file not found - page in Tomcat - (2009-03-18)
  [3996] Tips on Tomcat - moving applications around - (2013-02-05)

A502 - Web Application Deployment - Java - Basic Tools
  [1562] Java Compiler - needs all classes, compiles them all too - (2008-03-03)
  [1771] More HowTo diagrams - MySQL, Tomcat and Java - (2008-08-24)
  [2153] Class Loading and Variable Conversion in Java - (2009-05-02)
  [2422] Looking inside Java classes - javap and javadoc - (2009-09-25)
  [2859] Using java, javac, jar, and CLASSPATH - a simple example - (2010-07-07)
  [3151] Disassembling Python and Java - previously compiled code - (2011-01-29)
  [3819] Packing a tar, jar or war file - best practise - (2012-07-26)
  [4412] Java -making sure you have the right versions - (2015-02-02)

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