I created the jenkins project with ant build for java sources.
Install Warnings plugin, and add post-build action scan for compiler warnings.
To select parser for ant java, but there are no parser in the parser drop down list.
(screen capture attached.)

There are two javac parser, "Java Compiler(Eclipse)" and "Java Compiler(javac)".
I selected the later one, then no warning is reported in the ant build.

The console output sample is as follows:
[javac] Compiling 2 source files to C:\Users\momo\.jenkins\jobs\GoodMorningMrJenkins\workspace\app\Java8Lambdas\Album\build\classes
[javac] C:\Users\momo\.jenkins\jobs\GoodMorningMrJenkins\workspace\app\Java8Lambdas\Album\src\music\album\Artist.java:65: warning: [deprecation] getDate() in Date has been deprecated
[javac] int d = new Date().getDate();
[javac] ^
[javac] C:\Users\momo\.jenkins\jobs\GoodMorningMrJenkins\workspace\app\Java8Lambdas\Album\src\music\album\Artist.java:67: warning: [unchecked] unchecked call to add(E) as a member of the raw type List
[javac] list.add("Warning?");
[javac] ^
[javac] where E is a type-variable:
[javac] E extends Object declared in interface List
[javac] 2 warnings

Java Compiler(javac) parser cannot capture these warnings.

I found a AntJavaParser.java source file in Warnings plugin source files.
https://github.com/jenkinsci/warnings-plugin/blob/master/src/main/java/hudson/plugins/warnings/parser/AntJavacParser.java

This file seemed to define a proper regular expression for ant javac output.
It should be in parser seletion list like a "Java Compiler(Ant)".