tomcat＋jsp 报错，Unable to find a javac compiler;

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
	org.apache.tools.ant.taskdefs.compilers.CompilerAdapterFactory.getCompiler(CompilerAdapterFactory.java:106)
	org.apache.tools.ant.taskdefs.Javac.compile(Javac.java:935)
	org.apache.tools.ant.taskdefs.Javac.execute(Javac.java:764)
	org.apache.jasper.compiler.Compiler.generateClass(Compiler.java:382)
	org.apache.jasper.compiler.Compiler.compile(Compiler.java:472)
	org.apache.jasper.compiler.Compiler.compile(Compiler.java:451)
	org.apache.jasper.compiler.Compiler.compile(Compiler.java:439)
	org.apache.jasper.JspCompilationContext.compile(JspCompilationContext.java:511)
	org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:295)
	org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
	org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

note The full stack trace of the root cause is available in the Apache Tomcat/5.0.28 logs.