Unable to find a javac compiler

From EggeWiki

In Tomcat, Jetty, or another servlet container, you may receive an error stating the javac compiler can't be found. The obvious reason is if you are running from a JRE and not a JDK, however, it can still happen even when running a JDK.

The following is a typical error message:

HTTP ERROR: 500
 
Unable to find a javac compiler;
com.sun.tools.javac.Main is not on the classpath.
Perhaps JAVA_HOME does not point to the JDK.
It is currently set to "/java/jdk1.6.0_17/jre"
 
RequestURI=/web/
Caused by:
 
Unable to find a javac compiler;
com.sun.tools.javac.Main is not on the classpath.
Perhaps JAVA_HOME does not point to the JDK.
It is currently set to "/app/java/jdk1.6.0_17/jre"
	at org.apache.tools.ant.taskdefs.compilers.CompilerAdapterFactory.getCompiler(CompilerAdapterFactory.java:108)
	at org.apache.tools.ant.taskdefs.Javac.compile(Javac.java:1058)
	at org.apache.tools.ant.taskdefs.Javac.execute(Javac.java:882)
	at org.apache.jasper.compiler.Compiler.generateClass(Compiler.java:382)
	at org.apache.jasper.compiler.Compiler.compile(Compiler.java:472)
	at org.apache.jasper.compiler.Compiler.compile(Compiler.java:451)
	at org.apache.jasper.compiler.Compiler.compile(Compiler.java:439)
	at org.apache.jasper.JspCompilationContext.compile(JspCompilationContext.java:511)
	at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:295)
	at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
	at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
	at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:502)
	at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:363)
	at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
	at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181)
	at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:757)
	at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:416)
	at org.mortbay.jetty.servlet.Dispatcher.forward(Dispatcher.java:334)
	at org.mortbay.jetty.servlet.Dispatcher.forward(Dispatcher.java:126)
	at org.mortbay.jetty.servlet.DefaultServlet.doGet(DefaultServlet.java:463)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:707)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)

You can verify the running JVM properties using the jinfo command. Example:

/java/jdk1.6.0_17/bin/jinfo -sysprops 26314 | grep java.home
Attaching to process ID 26314, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 14.3-b01
java.home = /java/jdk1.6.0_17/jre

The easiest solution is to copy the tools.jar into the lib directory.

cp -p /java/jdk1.6.0_17/lib/tools.jar /java/jdk1.6.0_17/jre/lib/

Alternatively, you can specify tools.jar on the classpath. For example:

$ /java/jdk1.6.0_17/bin/java -cp /java/jdk1.6.0_17/lib/tools.jar com.sun.tools.javac.Main
Usage: javac <options> <source files>
where possible options include:
  -g                         Generate all debugging info
  -g:none                    Generate no debugging info
  -g:{lines,vars,source}     Generate only some debugging info
  -nowarn                    Generate no warnings
  -verbose                   Output messages about what the compiler is doing
  -deprecation               Output source locations where deprecated APIs are used
  -classpath <path>          Specify where to find user class files and annotation processors
  -cp <path>                 Specify where to find user class files and annotation processors
  -sourcepath <path>         Specify where to find input source files
  -bootclasspath <path>      Override location of bootstrap class files
  -extdirs <dirs>            Override location of installed extensions
  -endorseddirs <dirs>       Override location of endorsed standards path
  -proc:{none,only}          Control whether annotation processing and/or compilation is done.
  -processor <class1>[,<class2>,<class3>...]Names of the annotation processors to run; bypasses default discovery process
  -processorpath <path>      Specify where to find annotation processors
  -d <directory>             Specify where to place generated class files
  -s <directory>             Specify where to place generated source files
  -implicit:{none,class}     Specify whether or not to generate class files for implicitly referenced files 
  -encoding <encoding>       Specify character encoding used by source files
  -source <release>          Provide source compatibility with specified release
  -target <release>          Generate class files for specific VM version
  -version                   Version information
  -help                      Print a synopsis of standard options
  -Akey[=value]              Options to pass to annotation processors
  -X                         Print a synopsis of nonstandard options
  -J<flag>                   Pass <flag> directly to the runtime system