SLING-12834 Expose the cause of PersistenceException in its getMessage()

This is beneficial for cases where the stacktrace is not available (e.g.
in UI using PersistenceException.getMessage() or
PersistenceException.toString()) like SlingPostServlets HtmlResponse.

Author: kwin

src/main/java/org/apache/sling/api/resource/PersistenceException.java

@@ -19,6 +19,7 @@
 package org.apache.sling.api.resource;
 
 import java.io.IOException;
+import java.util.Set;
 
 /**
  * This exception will be thrown during the commit to persist
@@ -29,6 +30,8 @@ public class PersistenceException extends IOException {
 
     private static final long serialVersionUID = 2454225989618227698L;
 
+    private static final Set<Character> PUNCTUATION = Set.of('.', ':', '!');
+
     /** Optional resource path. */
     private final String resourcePath;
 
@@ -91,4 +94,34 @@ public String getResourcePath() {
     public String getPropertyName() {
         return this.propertyName;
     }
+
+    /**
+     * Returns a message for this exception.
+     * If the cause is not null, it will be appended to the message.
+     * @return The message for this exception
+     */
+    @Override
+    public String getMessage() {
+        StringBuilder sb = new StringBuilder();
+        if (this.getCause() != null) {
+            sb.append(stripTailingPunctuation(super.getMessage()))
+                    .append(" caused by ")
+                    .append(this.getCause().getMessage());
+        } else {
+            sb.append(super.getMessage());
+        }
+
+        return sb.toString();
+    }
+
+    private static String stripTailingPunctuation(String str) {
+        if (str == null || str.isEmpty()) {
+            return str;
+        }
+        int len = str.length();
+        if (len > 0 && PUNCTUATION.contains(str.charAt(len - 1))) {
+            return str.substring(0, len - 1);
+        }
+        return str;
+    }
 }

src/test/java/org/apache/sling/api/resource/PersistenceExceptionTest.java

@@ -0,0 +1,40 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.sling.api.resource;
+
+import javax.jcr.RepositoryException;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class PersistenceExceptionTest {
+
+    @Test
+    public void testGetMessage() {
+        PersistenceException e = new PersistenceException("Test message");
+        assertEquals("Test message", e.getMessage());
+        e = new PersistenceException("Test message", new RepositoryException("JCR error"));
+        assertEquals("Test message caused by JCR error", e.getMessage());
+        e = new PersistenceException("Test message!", new RepositoryException("JCR error"));
+        assertEquals("Test message caused by JCR error", e.getMessage());
+        e = new PersistenceException("Test message.", new RepositoryException("JCR error"));
+        assertEquals("Test message caused by JCR error", e.getMessage());
+    }
+}