Treat warnings as errors when compiling

See gh-21271

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/build.gradle

@@ -15,6 +15,7 @@ sourceSets {
 }
 
 dependencies {
+	testCompileOnly("com.google.code.findbugs:jsr305:3.0.2")
 	testImplementation(enforcedPlatform(project(":spring-boot-project:spring-boot-dependencies")))
 	testImplementation(project(":spring-boot-project:spring-boot-tools:spring-boot-test-support"))
 	testImplementation("javax.validation:validation-api")

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/build.gradle

@@ -65,3 +65,9 @@ processResources {
 	dependsOn reproducibleLoaderJar
 	dependsOn reproducibleJarModeLayerToolsJar
 }
+
+compileJava {
+	if ((!project.hasProperty("buildJavaHome")) && JavaVersion.current() == JavaVersion.VERSION_1_8) {
+		options.compilerArgs += ['-Xlint:-sunapi', '-XDenableSunApiLintControl']	
+	}	
+}