build.gradle

repositories {
    mavenCentral()
}

apply plugin: 'java'

sourceSets {
    main {
        java {
            srcDir '.'
            exclude 'Test*.java'
        }
    }
}

jar {
    manifest {
        attributes 'Main-Class': 'CheckCodeSource'
    }
}

task run(type: JavaExec) {
    main = project.hasProperty('main') ? project.getProperty('main') : 'CheckCodeSource'
    classpath = sourceSets.main.runtimeClasspath
    args 'main'
}

CheckCodeSource.java

import java.net.URL;
import java.security.CodeSource;
import java.security.ProtectionDomain;

public class CheckCodeSource {
    public static void main(final String[] args) throws SecurityException {
        final ProtectionDomain protectionDomain = CheckCodeSource.class.getProtectionDomain();
        System.out.println(">> ProtectionDomain:");
        System.out.println(protectionDomain.toString());

        final CodeSource codeSource = protectionDomain.getCodeSource();
        System.out.println(">> CodeSource:");
        System.out.println(codeSource.toString());

        final URL location = codeSource.getLocation();
        System.out.println(">> Location:");
        System.out.println(location.toString());
    }
}

Out of JAR:

$ ./gradlew run
:compileJava
:processResources UP-TO-DATE
:classes
:run
>> ProtectionDomain:
ProtectionDomain  (file:/some/workdir/CheckCodeSource/build/classes/main/ <no signer certificates>)
 sun.misc.Launcher$AppClassLoader@2a139a55
 <no principals>
 java.security.Permissions@4e25154f (
 ("java.io.FilePermission" "/some/workdir/CheckCodeSource/build/classes/main/-" "read")
 ("java.lang.RuntimePermission" "exitVM")
)


>> CodeSource:
(file:/some/workdir/CheckCodeSource/build/classes/main/ <no signer certificates>)
>> Location:
file:/some/workdir/CheckCodeSource/build/classes/main/

BUILD SUCCESSFUL

Total time: 0.821 secs

Inside JAR:

$ ./gradlew jar
:compileJava
:processResources
:classes
:jar

BUILD SUCCESSFUL

Total time: 0.785 secs

$ java -jar build/libs/CheckCodeSource.jar
>> ProtectionDomain:
ProtectionDomain  (file:/some/workdir/CheckCodeSource/build/libs/CheckCodeSource.jar <no signer certificates>)
 sun.misc.Launcher$AppClassLoader@5c647e05
 <no principals>
 java.security.Permissions@42a57993 (
 ("java.lang.RuntimePermission" "exitVM")
 ("java.io.FilePermission" "/some/workdir/CheckCodeSource/build/libs/CheckCodeSource.jar" "read")
)


>> CodeSource:
(file:/some/workdir/CheckCodeSource/build/libs/CheckCodeSource.jar <no signer certificates>)
>> Location:
file:/some/workdir/CheckCodeSource/build/libs/CheckCodeSource.jar