Use in process java compilation unless jdk is explicitly set by the developer

This PR updates KCT to use in process compilation unless the developer
explicitly provides a jdkHome that does not match the java home of the
current process.

In most cases, it won't be provided and because we have a default,
current implementation makes KCT spin off another process to compile java
sources, causing a significant compilation time penalty.
tschuchortdev/kotlin-compile-testing#113 (comment)

With this change, KCT won't create another process unless jdkHome is
set to a value that does not match the jdk home inherited from the
current process.

Issue: google#113
Test: As this is a bit difficult to test, I've added test that will
check the logs ¯\_(ツ)_/¯

Author: yigit

core/src/main/kotlin/com/tschuchort/compiletesting/KotlinCompilation.kt

@@ -170,7 +170,7 @@ class KotlinCompilation : AbstractKotlinCompilation<K2JVMCompilerArguments>() {
 	 * (on JDK8) or --system none (on JDK9+). This can be useful if all
 	 * the JDK classes you need are already on the (inherited) classpath.
 	 * */
-	var jdkHome: File? by default { getJdkHome() }
+	var jdkHome: File? by default { processJdkHome }
 
 	/**
 	 * Path to the kotlin-stdlib.jar
@@ -503,11 +503,11 @@ class KotlinCompilation : AbstractKotlinCompilation<K2JVMCompilerArguments>() {
 		if(javaSources.isEmpty())
 			return ExitCode.OK
 
-        if(jdkHome != null) {
+        if(jdkHome != null && jdkHome!!.canonicalPath != processJdkHome.canonicalPath) {
             /* If a JDK home is given, try to run javac from there so it uses the same JDK
                as K2JVMCompiler. Changing the JDK of the system java compiler via the
                "--system" and "-bootclasspath" options is not so easy. */
-
+            log("compiling java in a sub-process because a jdkHome is specified")
             val jdkBinFile = File(jdkHome, "bin")
             check(jdkBinFile.exists()) { "No JDK bin folder found at: ${jdkBinFile.toPath()}" }
 
@@ -531,22 +531,23 @@ class KotlinCompilation : AbstractKotlinCompilation<K2JVMCompilerArguments>() {
             }
         }
         else {
-            /*  If no JDK is given, we will use the host process' system java compiler
-                and erase the bootclasspath. The user is then on their own to somehow
+            /*  If no JDK is given, we will use the host process' system java compiler.
+                If it is set to `null`, we will erase the bootclasspath. The user is then on their own to somehow
                 provide the JDK classes via the regular classpath because javac won't
                 work at all without them */
-
+			log("jdkHome is not specified. Using system java compiler of the host process.")
 			val isJavac9OrLater = isJdk9OrLater()
 			val javacArgs = baseJavacArgs(isJavac9OrLater).apply {
-				// erase bootclasspath or JDK path because no JDK was specified
-				if (isJavac9OrLater)
-					addAll("--system", "none")
-				else
-					addAll("-bootclasspath", "")
+				if (jdkHome == null) {
+				    log("jdkHome is set to null, removing booth classpath from java compilation")
+					// erase bootclasspath or JDK path because no JDK was specified
+					if (isJavac9OrLater)
+						addAll("--system", "none")
+					else
+						addAll("-bootclasspath", "")
+				}
 			}
 
-            log("jdkHome is null. Using system java compiler of the host process.")
-
             val javac = SynchronizedToolProvider.systemJavaCompiler
             val javaFileManager = javac.getStandardFileManager(null, null, null)
             val diagnosticCollector = DiagnosticCollector<JavaFileObject>()

core/src/main/kotlin/com/tschuchort/compiletesting/Utils.kt

@@ -19,11 +19,12 @@ internal fun getJavaHome(): File {
     return File(path).also { check(it.isDirectory) }
 }
 
-internal fun getJdkHome()
-    = if(isJdk9OrLater())
+internal val processJdkHome by lazy {
+    if(isJdk9OrLater())
         getJavaHome()
     else
         getJavaHome().parentFile
+}
 
 /** Checks if the JDK of the host process is version 9 or later */
 internal fun isJdk9OrLater(): Boolean

core/src/test/kotlin/com/tschuchort/compiletesting/KotlinCompilationTests.kt

@@ -7,10 +7,10 @@ import org.assertj.core.api.Assertions.assertThat
 import org.jetbrains.kotlin.compiler.plugin.AbstractCliOption
 import org.jetbrains.kotlin.compiler.plugin.CliOption
 import org.jetbrains.kotlin.compiler.plugin.CommandLineProcessor
-import org.jetbrains.kotlin.compiler.plugin.PluginCliOptionProcessingException
 import org.junit.Rule
 import org.junit.Test
 import org.junit.rules.TemporaryFolder
+import java.io.ByteArrayOutputStream
 import java.io.File
 import javax.annotation.processing.AbstractProcessor
 import javax.annotation.processing.RoundEnvironment
@@ -183,6 +183,9 @@ class KotlinCompilationTests {
 
 		assertThat(result.exitCode).isEqualTo(ExitCode.COMPILATION_ERROR)
 		assertThat(result.messages).contains("Unable to find package java.lang")
+		assertThat(result.messages).contains(
+			"jdkHome is set to null, removing booth classpath from java compilation"
+		)
 	}
 
 	@Test
@@ -808,6 +811,53 @@ class KotlinCompilationTests {
 		assertThat(result.outputDirectory.listFilesRecursively().map { it.name })
 				.contains("JSource.class", "KSource.class")
 	}
+
+	@Test
+	fun `java compilation runs in process when no jdk is specified`() {
+		val source = SourceFile.java("JSource.java",
+			"""
+				class JSource {}
+			""".trimIndent())
+		val result = defaultCompilerConfig().apply {
+			sources = listOf(source)
+		}.compile()
+		assertThat(result.exitCode).isEqualTo(ExitCode.OK)
+		assertThat(result.messages).contains(
+			"jdkHome is not specified. Using system java compiler of the host process."
+		)
+		assertThat(result.messages).doesNotContain(
+			"jdkHome is set to null, removing booth classpath from java compilation"
+		)
+	}
+
+	@Test
+	fun `java compilation runs in a sub-process when jdk is specified`() {
+		val source = SourceFile.java("JSource.java",
+			"""
+			class JSource {}
+			""".trimIndent())
+		val fakeJdkHome = temporaryFolder.newFolder("fake-jdk-home")
+		fakeJdkHome.resolve("bin").mkdirs()
+		val logsStream = ByteArrayOutputStream()
+		val compiler = defaultCompilerConfig().apply {
+			sources = listOf(source)
+			jdkHome = fakeJdkHome
+			messageOutputStream = logsStream
+		}
+		val compilation = runCatching {
+			// jdk is fake so it won't compile
+			compiler.compile()
+		}
+		// it should fail since we are passing a fake jdk
+		assertThat(compilation.isFailure).isTrue()
+		val logs = logsStream.toString(Charsets.UTF_8)
+		assertThat(logs).contains(
+			"compiling java in a sub-process because a jdkHome is specified"
+		)
+		assertThat(logs).doesNotContain(
+			"jdkHome is set to null, removing booth classpath from java compilation"
+		)
+	}
 
 	class InheritedClass {}
 }