DiagnosticListener will fail to handle the Null objects, Compiler API will fail when trying to access the object which is not initialized inside a DiagnosticListener.

Please see the following code
<code>
import javax.tools.*;

public class Foo {
    private StringBuilder sb;
    public void test(String... args) {
        JavaCompilerTool javac = ToolProvider.defaultJavaCompiler();
        DiagnosticListener dl = new DiagnosticListener() {
            public void report(DiagnosticMessage message) {
                sb.append("test");
            }
        };
        javac.run((JavaFileManager)null, dl, null, args);
    }
    public static void main(String... arg) {
        Foo test = new Foo();
        test.test("Test6.java");
    }
}
</code>
In the above code, sb is not initialized and trying to access inside DiagnosticListener.

Output of the code when compiled and run
<Output>
bash-3.00$ javac -Xlint Foo.java
bash-3.00$ java Foo
An exception has occurred in the compiler (1.6.0-ea). Please file a bug at the Java Developer Connection (http://java.sun.com/webapps/bugreport) after checking the Bug Parade for duplicates. Include your program and the following diagnostic in your report. Thank you.
java.lang.NullPointerException
        at Foo$1.report(Foo.java:9)
        at com.sun.tools.javac.util.Log.writeDiagnostic(Log.java:444)
        at com.sun.tools.javac.util.Log.report(Log.java:432)
        at com.sun.tools.javac.util.Log.error(Log.java:363)
        at com.sun.tools.javac.comp.Resolve$ResolveError.report(Resolve.java:1576)
        at com.sun.tools.javac.comp.Resolve.access(Resolve.java:1053)
        at com.sun.tools.javac.comp.Resolve.access(Resolve.java:1073)
        at com.sun.tools.javac.comp.Resolve.resolveIdent(Resolve.java:1136)
        at com.sun.tools.javac.comp.Attr.visitIdent(Attr.java:1625)
        at com.sun.tools.javac.tree.Tree$Ident.accept(Tree.java:1251)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:277)
        at com.sun.tools.javac.comp.Attr.attribType(Attr.java:305)
        at com.sun.tools.javac.comp.MemberEnter.visitVarDef(MemberEnter.java:604)
        at com.sun.tools.javac.tree.Tree$VarDef.accept(Tree.java:592)
        at com.sun.tools.javac.comp.MemberEnter.memberEnter(MemberEnter.java:385)
        at com.sun.tools.javac.comp.Attr.visitVarDef(Attr.java:556)
        at com.sun.tools.javac.tree.Tree$VarDef.accept(Tree.java:592)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:277)
        at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:312)
        at com.sun.tools.javac.comp.Attr.attribStats(Attr.java:328)
        at com.sun.tools.javac.comp.Attr.visitBlock(Attr.java:626)
        at com.sun.tools.javac.tree.Tree$Block.accept(Tree.java:630)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:277)
        at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:312)
        at com.sun.tools.javac.comp.Attr.visitTry(Attr.java:792)
        at com.sun.tools.javac.tree.Tree$Try.accept(Tree.java:808)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:277)
        at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:312)
        at com.sun.tools.javac.comp.Attr.attribStats(Attr.java:328)
        at com.sun.tools.javac.comp.Attr.visitBlock(Attr.java:626)
        at com.sun.tools.javac.tree.Tree$Block.accept(Tree.java:630)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:277)
        at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:312)
        at com.sun.tools.javac.comp.Attr.visitMethodDef(Attr.java:537)
        at com.sun.tools.javac.tree.Tree$MethodDef.accept(Tree.java:545)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:277)
        at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:312)
        at com.sun.tools.javac.comp.Attr.attribClassBody(Attr.java:2525)
        at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:2457)
        at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:2393)
        at com.sun.tools.javac.main.JavaCompiler.attribute(JavaCompiler.java:636)
        at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:549)
        at com.sun.tools.javac.main.Main.compile(Main.java:680)
        at com.sun.tools.javac.main.Main.compile(Main.java:611)
        at com.sun.tools.javac.api.Tool.runImpl(Tool.java:122)
        at com.sun.tools.javac.api.Tool.run(Tool.java:131)
        at Foo.test(Foo.java:12)
        at Foo.main(Foo.java:16)

</Output>