The method openReader throws a documented, but unexpected UnsupportedOperationException exception.

see a test:
------------
import javax.tools.*;
import java.io.*;
import java.util.Collections;

public class test9 {

    public static void main(String[] argv){
        JavaCompilerTool compiler = ToolProvider.getSystemJavaCompilerTool();
        StandardJavaFileManager mgr = compiler.getStandardFileManager( new DiagnosticCollector<JavaFileObject>() );
        System.out.println( new File( new File(".").toURI() ).getAbsolutePath() );
        mgr.setLocation(StandardJavaFileManager.StandardLocation.SOURCE_PATH,
                            Collections.singleton(new File( new File(".").toURI())));

        try {
            JavaFileObject f = mgr.getJavaFileForInput(
                        StandardJavaFileManager.StandardLocation.SOURCE_PATH, "test9",
                        JavaFileObject.Kind.SOURCE );
            f.openReader(true);
        } catch( Exception x ){
            x.printStackTrace(System.out);
        }
        System.out.println( "The test completed successfully if there are no error messages in the output." );
    }
}
------------
result:
------------
Z:\tests>z:/lnks/jdk6/bin/java.exe -cp . test9
Z:\tests\.
java.lang.UnsupportedOperationException
    at com.sun.tools.javac.util.DefaultFileManager$RegularFileObject.openReader(DefaultFileManager.java:1042)
    at test9.main(test9.java:18)
The test completed successfully if there are no error messages in the output.
------------
Simplified testcase :

import java.io.BufferedReader;
import javax.tools.*;
public class T6427274 {
    public static void main(String... arg) throws Throwable {
        JavaCompiler javac = ToolProvider.getSystemJavaCompiler();
        StandardJavaFileManager jfm = javac.getStandardFileManager(null,null,null);
        Iterable<? extends JavaFileObject> jfos = jfm.getJavaFileObjects("T6427274.java");
        JavaFileObject jfo = jfos.iterator().next();
        BufferedReader in = new BufferedReader(jfo.openReader(true));
// thows java.lang.UnsupportedOperationException
    }
}