Gradle compileJava task warning: [options] bootstrap class path not set in conjunction with -source 1.6

Question

Below is the content of the build.gradle file:

apply plugin: 'java'

archivesBaseName    = 'foo-bar'
version             = '1.0'
sourceCompatibility = JavaVersion.VERSION_1_6
targetCompatibility = JavaVersion.VERSION_1_6

sourceSets.main.resources.exclude 'foo.jks'

compileJava{
    println project.sourceCompatibility 
    println project.targetCompatibility 
    println sourceCompatibility 
    println targetCompatibility 
}

And below is the result of running the Gradle jar task:

[sts] -----------------------------------------------------
[sts] Starting Gradle build for the following tasks: 
[sts]      :jar
[sts] -----------------------------------------------------
1.6
1.6
1.6
1.6
:compileJavawarning: [options] bootstrap class path not set in conjunction with -source 1.6
1 warning

:processResources UP-TO-DATE
:classes
:jar

BUILD SUCCESSFUL

Total time: 1 mins 3.072 secs
[sts] -----------------------------------------------------
[sts] Build finished succesfully!
[sts] Time taken: 1 min, 3 sec
[sts] -----------------------------------------------------

Although it claims that the build was successful, the warning has me a little worried about run-time complications. How do I resolve this warning?

Also, I'm using Eclipse Indigo. And I have the Java compiler and build path of my project set to jdk1.6.0_35.

If anyone needs more information, please let me know!

---

UPDATE

I navigated to Window > Preferences > Gradle and I set "Java Home" to the "Workspace JRE" jdk1.6.0_35,

And now I get the following output when I run the Gradle jar task,

[sts] -----------------------------------------------------
[sts] Starting Gradle build for the following tasks: 
[sts]      :jar
[sts] -----------------------------------------------------
1.6
1.6
1.6
1.6
:compileJavawarning: java\lang\Enum.class(java\lang:Enum.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\lang\Comparable.class(java\lang:Comparable.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\lang\Object.class(java\lang:Object.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\io\Serializable.class(java\io:Serializable.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\lang\String.class(java\lang:String.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\io\InputStream.class(java\io:InputStream.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\security\KeyStore.class(java\security:KeyStore.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: javax\crypto\Cipher.class(javax\crypto:Cipher.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: javax\crypto\spec\SecretKeySpec.class(javax\crypto\spec:SecretKeySpec.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: javax\xml\bind\DatatypeConverter.class(javax\xml\bind:DatatypeConverter.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\lang\CloneNotSupportedException.class(java\lang:CloneNotSupportedException.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\lang\Class.class(java\lang:Class.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\lang\ClassLoader.class(java\lang:ClassLoader.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\security\KeyStore$LoadStoreParameter.class(java\security:KeyStore$LoadStoreParameter.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\io\Closeable.class(java\io:Closeable.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\lang\AutoCloseable.class(java\lang:AutoCloseable.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\lang\Exception.class(java\lang:Exception.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\lang\Throwable.class(java\lang:Throwable.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\security\cert\Certificate.class(java\security\cert:Certificate.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\security\Key.class(java\security:Key.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\security\spec\KeySpec.class(java\security\spec:KeySpec.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: javax\crypto\SecretKey.class(javax\crypto:SecretKey.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\lang\Error.class(java\lang:Error.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\security\KeyStoreException.class(java\security:KeyStoreException.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\security\GeneralSecurityException.class(java\security:GeneralSecurityException.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\lang\RuntimeException.class(java\lang:RuntimeException.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\io\IOException.class(java\io:IOException.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\security\NoSuchAlgorithmException.class(java\security:NoSuchAlgorithmException.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\security\cert\CertificateException.class(java\security\cert:CertificateException.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: javax\crypto\NoSuchPaddingException.class(javax\crypto:NoSuchPaddingException.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\security\InvalidKeyException.class(java\security:InvalidKeyException.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\security\KeyException.class(java\security:KeyException.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\nio\ByteBuffer.class(java\nio:ByteBuffer.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: javax\crypto\IllegalBlockSizeException.class(javax\crypto:IllegalBlockSizeException.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: javax\crypto\BadPaddingException.class(javax\crypto:BadPaddingException.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\lang\StringBuilder.class(java\lang:StringBuilder.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\lang\AbstractStringBuilder.class(java\lang:AbstractStringBuilder.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\lang\CharSequence.class(java\lang:CharSequence.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
warning: java\lang\StringBuffer.class(java\lang:StringBuffer.class): major version 51 is newer than 50, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
39 warnings

:processResources UP-TO-DATE
:classes
:jar

BUILD SUCCESSFUL

Total time: 1.96 secs
[sts] -----------------------------------------------------
[sts] Build finished succesfully!
[sts] Time taken: 0 min, 1 sec
[sts] -----------------------------------------------------

What do?

Answer 1

You can bootstrap class path using bootClasspath option:

apply plugin: 'java'

sourceCompatibility = 1.6
targetCompatibility = 1.6

compileJava.options.bootClasspath = "$JDK6_HOME/jre/lib/rt.jar"

To set the bootClasspath option on all compile tasks in the project you can use the withType() method on the TaskContainer to find all tasks of type Compile:

apply plugin: 'java'

sourceCompatibility = 1.6
targetCompatibility = 1.6

tasks.withType(JavaCompile) {
    options.bootstrapClasspath = files("$JDK6_HOME/jre/lib/rt.jar")
}

gradle.properties:

JDK6_HOME=C:/JAVA/jdk6

See documentation for details.

Answer 2

See the javac docs on cross compilation for details but basically it means you can compile against jdk classes that don't exist, or were different, on your target version. For example perhaps you use java.util.Deque but are targeting jdk5.

I don't believe gradle has built in support for setting this. I have found that you need to twiddle the compile task manually. For example

def bootClasspathStr = "${yourJavaVersionXInstallationPath}/jre/lib/rt.jar"
project.tasks.withType(AbstractCompile, { AbstractCompile ac ->
    ac.options.bootClasspath = bootClasspathStr // options is always there but not defined on AbstractCompile so going to hit it anyway
})

Having said all that, you appear to be building on jdk6 for java6 so I would think you can safely ignore the warning. Are you sure gradle is running under jdk6 and not 7?