[fixes #3143] Maven+ECJ Agent Bootstrap

* Adds AgentBootstrap, the actual bootstrapping agent
* Adds MavenEcjBootstrapApp, the command line app for generating the
appropriate files
* Updates the build to package these correctly
* Updates the documentation for setup/ecj

1 files changed, 92 insertions, 0 deletions

diff --git a/src/eclipseAgent/lombok/launch/AgentBootstrap.java b/src/eclipseAgent/lombok/launch/AgentBootstrap.java
new file mode 100644
index 00000000..bbd9aa4d
--- /dev/null
+++ b/src/eclipseAgent/lombok/launch/AgentBootstrap.java
@@ -0,0 +1,92 @@
+/*
+ * Copyright (C) 2009-2022 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 AbstractCompilerMojo class that compiler instances extend.
+ * <li>Maven loaders are ClassRealms, which extend URLClassLoader.
+ * <li>Each plugin dependency in the pom.xml 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 ([...]/groupid/artifactid/ver/artifactid-ver.jar).
+ * <li>The Lombok Java agent class is lombok.launch.Agent.
+ * </ul>
+ * Given all of the above, the transformer simply waits for 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 AgentBootstrap
+{
+    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 AgentBootstrap()
+    {}
+
+    /**
+     * Invoked when agent is added via -javaagent argument.
+     *
+     * @param agentArgs
+     *            arguments
+     * @param instrumentation
+     *            service provider
+     */
+    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()));
+                                AgentBootstrap.class.getClassLoader().loadClass(LOMBOK_AGENT_CLASS)
+                                        .getDeclaredMethod("premain", String.class, Instrumentation.class)
+                                        .invoke(null, agentArgs, instrumentation);
+                                instrumentation.removeTransformer(this);
+                                break;
+                            } catch (final Exception e) {
+                                //There are no appropriate loggers available at this point in time.
+                                e.printStackTrace(System.err);
+                            }
+                        }
+                    }
+                }
+                return NOT_TRANSFORMED;
+            }
+        });
+    }
+}