JPA ClassFileTransformerAdapter defensively backs out when called from within a transform call

Issue: SPR-13886

Author: jhoeller

spring-orm/src/main/java/org/springframework/orm/jpa/persistenceunit/ClassFileTransformerAdapter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -27,19 +27,23 @@
 
 /**
  * Simple adapter that implements the {@code java.lang.instrument.ClassFileTransformer}
- * interface based on a JPA ClassTransformer which a JPA PersistenceProvider asks the
- * PersistenceUnitInfo to install in the current runtime.
+ * interface based on a JPA {@code ClassTransformer} which a JPA PersistenceProvider
+ * asks the {@code PersistenceUnitInfo} to install in the current runtime.
  *
  * @author Rod Johnson
+ * @author Juergen Hoeller
  * @since 2.0
  * @see javax.persistence.spi.PersistenceUnitInfo#addTransformer(javax.persistence.spi.ClassTransformer)
  */
 class ClassFileTransformerAdapter implements ClassFileTransformer {
 
 	private static final Log logger = LogFactory.getLog(ClassFileTransformerAdapter.class);
 
+
 	private final ClassTransformer classTransformer;
 
+	private boolean currentlyTransforming = false;
+
 
 	public ClassFileTransformerAdapter(ClassTransformer classTransformer) {
 		Assert.notNull(classTransformer, "ClassTransformer must not be null");
@@ -52,28 +56,42 @@ public byte[] transform(
 			ClassLoader loader, String className, Class<?> classBeingRedefined,
 			ProtectionDomain protectionDomain, byte[] classfileBuffer) {
 
-		try {
-			byte[] transformed = this.classTransformer.transform(
-					loader, className, classBeingRedefined, protectionDomain, classfileBuffer);
-			if (transformed != null && logger.isDebugEnabled()) {
-				logger.debug("Transformer of class [" + this.classTransformer.getClass().getName() +
-						"] transformed class [" + className + "]; bytes in=" +
-						classfileBuffer.length + "; bytes out=" + transformed.length);
+		synchronized (this) {
+			if (this.currentlyTransforming) {
+				// Defensively back out when called from within the transform delegate below:
+				// in particular, for the over-eager transformer implementation in Hibernate 5.
+				return null;
 			}
-			return transformed;
-		}
-		catch (ClassCircularityError ex) {
-			logger.error("Error weaving class [" + className + "] with " +
-					"transformer of class [" + this.classTransformer.getClass().getName() + "]", ex);
-			throw new IllegalStateException("Could not weave class [" + className + "]", ex);
-		}
-		catch (Throwable ex) {
-			if (logger.isWarnEnabled()) {
-				logger.warn("Error weaving class [" + className + "] with " +
-						"transformer of class [" + this.classTransformer.getClass().getName() + "]", ex);
+
+			this.currentlyTransforming = true;
+			try {
+				byte[] transformed = this.classTransformer.transform(
+						loader, className, classBeingRedefined, protectionDomain, classfileBuffer);
+				if (transformed != null && logger.isDebugEnabled()) {
+					logger.debug("Transformer of class [" + this.classTransformer.getClass().getName() +
+							"] transformed class [" + className + "]; bytes in=" +
+							classfileBuffer.length + "; bytes out=" + transformed.length);
+				}
+				return transformed;
+			}
+			catch (ClassCircularityError ex) {
+				if (logger.isErrorEnabled()) {
+					logger.error("Circularity error while weaving class [" + className + "] with " +
+							"transformer of class [" + this.classTransformer.getClass().getName() + "]", ex);
+				}
+				throw new IllegalStateException("Failed to weave class [" + className + "]", ex);
+			}
+			catch (Throwable ex) {
+				if (logger.isWarnEnabled()) {
+					logger.warn("Error weaving class [" + className + "] with transformer of class [" +
+							this.classTransformer.getClass().getName() + "]", ex);
+				}
+				// The exception will be ignored by the class loader, anyway...
+				throw new IllegalStateException("Could not weave class [" + className + "]", ex);
+			}
+			finally {
+				this.currentlyTransforming = false;
 			}
-			// The exception will be ignored by the class loader, anyway...
-			throw new IllegalStateException("Could not weave class [" + className + "]", ex);
 		}
 	}