[#3143] Review and update jopatai's work on the maven/ecj agent jar

* Update copyright headers
* Update code style (tabs, not spaces, spaces around + operator - that's about it)
* Use `x.class.getResourceAsStream`, not `x.getClass().` - minor mostly irrelevant nit.
* Rename and re-locate the jar itself.
* 'ecj' as an alias for this command seems a bit too cavalier' removed it.
* The source is in its own 'root' src dir, it doesn't really fit in the eclipse agent sources - it's more a maven agent.
* Fixed a bug where a filehandle wasn't safe closed. Mostly irrelevant (JVM would quite afterwards anyway).
* Slight rewording of the ecj docs.

1 files changed, 83 insertions, 0 deletions

diff --git a/src/mavenEcjBootstrapAgent/lombok/launch/MavenEcjBootstrapAgent.java b/src/mavenEcjBootstrapAgent/lombok/launch/MavenEcjBootstrapAgent.java
new file mode 100644
index 00000000..b36e591f
--- /dev/null
+++ b/src/mavenEcjBootstrapAgent/lombok/launch/MavenEcjBootstrapAgent.java
@@ -0,0 +1,83 @@
+/*
+ * Copyright (C) 2022 The Project Lombok Authors.
+ * 
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ * 
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ * 
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package lombok.launch;
+
+import java.lang.instrument.ClassFileTransformer;
+import java.lang.instrument.IllegalClassFormatException;
+import java.lang.instrument.Instrumentation;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.security.ProtectionDomain;
+import java.util.jar.JarFile;
+
+/**
+ * This Java agent does not transform bytecode, but acts as a watcher that can
+ * figure out when it is appropriate to load Lombok itself within a Maven
+ * execution.
+ * 
+ * It relies on several facts:
+ * <ul>
+ * <li>maven-compiler-plugin contains an {@code AbstractCompilerMojo} class that
+ * compiler instances extend.
+ * <li>Maven loaders are {@code ClassRealms}, which extend {@code URLClassLoader}.
+ * <li>Each plugin dependency in the <em>pom.xml </em>is represented as a file URL on the
+ * ClassRealm that points to the artifact.
+ * <li>URLs to Maven artifacts contain the group and artifact ids
+ * ({@code [...]/groupid/artifactid/ver/artifactid-ver.jar}).
+ * <li>The Lombok Java agent class is {@code lombok.launch.Agent}.
+ * </ul>
+ * Given all of the above, the transformer simply waits for {@code AbstractCompilerMojo}
+ * to be loaded, then uses the loader to find the path to the Lombok jar file,
+ * and finally loads the Lombok agent using reflection.
+ */
+public final class MavenEcjBootstrapAgent {
+	private static final String MAVEN_COMPILER_TRIGGER_CLASS = "org/apache/maven/plugin/compiler/AbstractCompilerMojo";
+	private static final String LOMBOK_URL_IDENTIFIER = "/org/projectlombok/lombok/";
+	private static final String LOMBOK_AGENT_CLASS = "lombok.launch.Agent";
+	private static final byte[] NOT_TRANSFORMED = null;
+	
+	private MavenEcjBootstrapAgent() {}
+	
+	public static void premain(final String agentArgs, final Instrumentation instrumentation) {
+		instrumentation.addTransformer(new ClassFileTransformer() {
+			@Override public byte[] transform(final ClassLoader loader, final String className, final Class<?> cbr, final ProtectionDomain pd, final byte[] cfb) throws IllegalClassFormatException {
+				if (MAVEN_COMPILER_TRIGGER_CLASS.equals(className)) {
+					for (final URL url : ((URLClassLoader) loader).getURLs()) {
+						if (url.getPath().contains(LOMBOK_URL_IDENTIFIER)) {
+							try {
+								instrumentation.appendToSystemClassLoaderSearch(new JarFile(url.getPath()));
+								MavenEcjBootstrapAgent.class.getClassLoader().loadClass(LOMBOK_AGENT_CLASS).getDeclaredMethod("premain", String.class, Instrumentation.class).invoke(null, agentArgs, instrumentation);
+								instrumentation.removeTransformer(this);
+								break;
+							} catch (Exception e) {
+								// There are no appropriate loggers available at
+								// this point in time.
+								e.printStackTrace(System.err);
+							}
+						}
+					}
+				}
+				return NOT_TRANSFORMED;
+			}
+		});
+	}
+}