FULL PRODUCT VERSION :
java version "1.7.0_05"
Java(TM) SE Runtime Environment (build 1.7.0_05-b05)
Java HotSpot(TM) Client VM (build 23.1-b03, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Windows 7 32 bits
Microsoft Windows [version 6.1.7601]


A DESCRIPTION OF THE PROBLEM :
I use the jre and I have included tools.jar in my classpath

  ToolProvider.getSystemJavaCompiler() returns null;

This code works and gives me a compiler :
compiler = (JavaCompiler) Class.forName("com.sun.tools.javac.api.JavacTool").newInstance();

Note : It works with java 6 versions .

REGRESSION. Last worked in version 6u31

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Use a jre, include tools.jar from the jdk in your classpath
execute ToolProvider.getSystemJavaCompiler()
it returns null

execute (JavaCompiler) Class.forName("com.sun.tools.javac.api.JavacTool").newInstance();
it returns a compiler

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
  ToolProvider.getSystemJavaCompiler() should return a compiler
ACTUAL -
it returns null

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------

import javax.tools.JavaCompiler;
import javax.tools.ToolProvider;

public class Test {

public static void main(String[] args) {
try {
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
if (compiler == null) {
System.out.println("ToolProvider.getSystemJavaCompiler() returned null");
compiler = (JavaCompiler) Class.forName("com.sun.tools.javac.api.JavacTool").newInstance();
System.out.println("But here is a compiler : " + compiler);
}
} catch (Exception e) {
e.printStackTrace();
}
}

}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
JavaCompiler compiler = (JavaCompiler) Class.forName("com.sun.tools.javac.api.JavacTool").newInstance();