Allow instrumentation to be used with Attach API

Update InstrumentationSavingAgent to allow it to be used with the
Attach API

Issue: SPR-11083

build.gradle

@@ -327,6 +327,8 @@ project("spring-instrument") {
 	jar {
 		manifest.attributes["Premain-Class"] =
 			"org.springframework.instrument.InstrumentationSavingAgent"
+		manifest.attributes["Agent-Class"] =
+			"org.springframework.instrument.InstrumentationSavingAgent"
 		manifest.attributes["Can-Redefine-Classes"] = "true"
 		manifest.attributes["Can-Retransform-Classes"] = "true"
 		manifest.attributes["Can-Set-Native-Method-Prefix"] = "false"

spring-instrument/src/main/java/org/springframework/instrument/InstrumentationSavingAgent.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2013 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -39,6 +39,13 @@ public static void premain(String agentArgs, Instrumentation inst) {
 		instrumentation = inst;
 	}
 
+	/**
+	 * Save the {@link Instrumentation} interface exposed by the JVM.
+	 * This method is required to dynamically load this Agent with the Attach API.
+	 */
+	public static void agentmain(String agentArgs, Instrumentation inst) {
+		instrumentation = inst;
+	}
 
 	/**
 	 * Return the {@link Instrumentation} interface exposed by the JVM.
@@ -48,8 +55,9 @@ public static void premain(String agentArgs, Instrumentation inst) {
 	 * {@link org.springframework.instrument.classloading.InstrumentationLoadTimeWeaver#getInstrumentation()}
 	 * instead - which will work without the agent class in the classpath as well.
 	 * @return the {@code Instrumentation} instance previously saved when
-	 * the {@link #premain} method was called by the JVM; will be {@code null}
-	 * if this class was not used as Java agent when this JVM was started.
+	 * the {@link #premain} or {@link #agentmain} methods was called by the JVM;
+	 * will be {@code null} if this class was not used as Java agent when this
+	 * JVM was started or it wasn't installed as agent using the Attach API.
 	 * @see org.springframework.instrument.classloading.InstrumentationLoadTimeWeaver#getInstrumentation()
 	 */
 	public static Instrumentation getInstrumentation() {