Support incremental annotation processing with Gradle

Closes gh-22150

spring-boot-project/spring-boot-tools/spring-boot-autoconfigure-processor/src/main/resources/META-INF/gradle/incremental.annotation.processors

+org.springframework.boot.autoconfigureprocessor.AutoConfigureAnnotationProcessor,aggregating
\ No newline at end of file

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/javac/Trees.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -16,6 +16,7 @@
 
 package org.springframework.boot.configurationprocessor.fieldvalues.javac;
 
+import java.lang.reflect.Field;
 import java.lang.reflect.Method;
 
 import javax.annotation.processing.ProcessingEnvironment;
@@ -38,10 +39,21 @@ final class Trees extends ReflectionWrapper {
 	}
 
 	static Trees instance(ProcessingEnvironment env) throws Exception {
-		ClassLoader classLoader = env.getClass().getClassLoader();
-		Class<?> type = findClass(classLoader, "com.sun.source.util.Trees");
-		Method method = findMethod(type, "instance", ProcessingEnvironment.class);
-		return new Trees(method.invoke(null, env));
+		try {
+			ClassLoader classLoader = env.getClass().getClassLoader();
+			Class<?> type = findClass(classLoader, "com.sun.source.util.Trees");
+			Method method = findMethod(type, "instance", ProcessingEnvironment.class);
+			return new Trees(method.invoke(null, env));
+		}
+		catch (Exception ex) {
+			return instance(unwrap(env));
+		}
+	}
+
+	private static ProcessingEnvironment unwrap(ProcessingEnvironment wrapper) throws Exception {
+		Field delegateField = wrapper.getClass().getDeclaredField("delegate");
+		delegateField.setAccessible(true);
+		return (ProcessingEnvironment) delegateField.get(wrapper);
 	}
 
 }

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/src/main/resources/META-INF/gradle/incremental.annotation.processors

+org.springframework.boot.configurationprocessor.ConfigurationMetadataAnnotationProcessor,aggregating
\ No newline at end of file