feat: support earlier transformation as -javaagent

Signed-off-by: Joke de Buhr <joke@xckk.de>

README.adoc

@@ -35,6 +35,11 @@ image:https://badgen.net/github/release/joke/spock-mockable/stable[]
 dependencies {
     testImplementation 'io.github.joke:spock-mockable:x.y.z'
 }
+
+// to load the agent even earlier add it as a JVM argument
+tasks.withType(Test) {
+    jvmArgs += ["-javaagent:${classpath.find { it.name.contains('spock-mockable') }.absolutePath}"]
+}
 ----
 
 === Maven
@@ -119,6 +124,8 @@ At the start of a test JVM these detected classes are transformed by the JVM ins
 regardless of the actual specification there the mock invocation has been detected.
 This might lead to unexpected behaviour between different specifications.
 
+For the earliest possible transformation of classes start the agent by using the JVM argument (`-javaagent`).
+
 IMPORTANT: For agent instrumentation to work the JVM must support this feature. A JRE is most likely not sufficient.
 
 IMPORTANT: The agent is attached to the JVM as early as possible but some classes not be transformed nevertheless because they are used prior. This restriction applies to some `java.lang` classes but also to some junit classes.

spock-mockable/build.gradle

@@ -42,6 +42,16 @@ shadowJar {
     minimize()
 }
 
+jar {
+    manifest {
+        attributes(
+                'Can-Redefine-Classes': 'true',
+                'Can-Retransform-Classes': 'true',
+                'Premain-Class': 'io.github.joke.spockmockable.agent.AgentInstaller',
+        )
+    }
+}
+
 publishing {
     publications {
         spockMockable(MavenPublication) { publication ->

spock-mockable/src/main/java/io/github/joke/spockmockable/agent/AgentInstaller.java

@@ -10,6 +10,7 @@
 
 import javax.inject.Singleton;
 import java.lang.instrument.Instrumentation;
+import java.util.Optional;
 
 import static java.lang.Boolean.parseBoolean;
 import static java.lang.System.getProperty;
@@ -20,10 +21,19 @@
 @Component(modules = AgentInstaller.Module.class)
 public abstract class AgentInstaller {
 
+    @Nullable
+    private static Instrumentation instrumentation;
+
     @SuppressWarnings("SameNameButDifferent")
     @Getter(value = PRIVATE, lazy = true, onMethod_ = @Nullable)
     private static final AgentInstaller agentInstaller = DaggerAgentInstaller.create().init();
 
+    @SuppressWarnings("DoNotCallSuggester")
+    public static void premain(String agentArgs, Instrumentation instrumentation) {
+        AgentInstaller.instrumentation = instrumentation;
+        install();
+    }
+
     @Synchronized
     public static void install() {
         getAgentInstaller();
@@ -57,7 +67,8 @@ interface Module {
         @Provides
         @Singleton
         static Instrumentation getInstrumentation() {
-            return ByteBuddyAgent.install();
+            return Optional.ofNullable(AgentInstaller.instrumentation)
+                    .orElseGet(ByteBuddyAgent::install);
         }
 
     }