Add gradle task to verify bytecode version of compiled classes (#2954)

Author: MinnDevelopment

build.gradle.kts

@@ -17,6 +17,7 @@
 import com.github.benmanes.gradle.versions.updates.DependencyUpdatesTask
 import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
 import de.undercouch.gradle.tasks.download.Download
+import net.dv8tion.jda.tasks.VerifyBytecodeVersion
 import net.dv8tion.jda.tasks.Version
 import net.dv8tion.jda.tasks.applyAudioExclusions
 import net.dv8tion.jda.tasks.applyOpusExclusions
@@ -327,7 +328,7 @@ tasks.withType<JavaCompile> {
     options.compilerArgs.addAll(args)
 }
 
-tasks.named<JavaCompile>("compileJava").configure {
+val compileJava by tasks.getting(JavaCompile::class) {
     dependsOn(generateJavaSources)
     source = generateJavaSources.get().source
 }
@@ -393,6 +394,17 @@ tasks.test {
     }
 }
 
+val verifyBytecodeVersion by tasks.registering(VerifyBytecodeVersion::class) {
+    group = "verification"
+
+    expectedMajorVersion = 52
+    classes.from(compileJava.outputs.files.asFileTree.matching {
+        include("**/*.class")
+    })
+}
+
+compileJava.finalizedBy(verifyBytecodeVersion)
+
 
 ////////////////////////////////////
 //                                //

buildSrc/src/main/kotlin/net/dv8tion/jda/tasks/VerifyBytecodeVersion.kt

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2015 Austin Keener, Michael Ritter, Florian Spieß, and the JDA contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package net.dv8tion.jda.tasks
+
+import org.gradle.api.DefaultTask
+import org.gradle.api.GradleException
+import org.gradle.api.file.ConfigurableFileCollection
+import org.gradle.api.provider.Property
+import org.gradle.api.tasks.CacheableTask
+import org.gradle.api.tasks.Input
+import org.gradle.api.tasks.InputFiles
+import org.gradle.api.tasks.PathSensitive
+import org.gradle.api.tasks.PathSensitivity
+import org.gradle.api.tasks.TaskAction
+import java.io.File
+
+@CacheableTask
+abstract class VerifyBytecodeVersion : DefaultTask() {
+    @get:InputFiles
+    @get:PathSensitive(PathSensitivity.RELATIVE)
+    abstract val classes: ConfigurableFileCollection
+
+    @get:Input
+    abstract val expectedMajorVersion: Property<Int>
+
+    @TaskAction
+    fun verify() {
+        val badFiles = classes.filter { it.readBytecodeVersion() != expectedMajorVersion.get() }.toList()
+
+        if (badFiles.isNotEmpty()) {
+            println("Found ${badFiles.size} files that did not have the bytecode version ${expectedMajorVersion.get()}")
+            println("The following classes have the wrong bytecode version:")
+            for (file in badFiles) {
+                println("\t- ${file.path} has version ${file.readBytecodeVersion()}")
+            }
+
+            throw GradleException("Some compiled classes have the wrong bytecode version.")
+        }
+    }
+}
+
+private val validClassHeaderBytes = listOf(0xCA.toByte(), 0xFE.toByte(), 0xBA.toByte(), 0xBE.toByte())
+
+private fun File.readBytecodeVersion(): Int? = inputStream().buffered().use { stream ->
+    val header = stream.readNBytes(8)
+    if (header.size < 8) {
+        return null
+    }
+
+    if (header.take(4) != validClassHeaderBytes) {
+        return null
+    }
+
+    // Big-endian major version: bytes[6..7]
+    return ((header[6].toInt() and 0xFF) shl 8) or (header[7].toInt() and 0xFF)
+}