8218628: Add detailed message to NullPointerException describing what is null

This is the implementation of JEP 358: Helpful NullPointerExceptions.

Reviewed-by: coleenp, clanger, rschmelter, rriggs, forax, mr

src/java.base/share/classes/java/lang/NullPointerException.java

@@ -70,4 +70,35 @@ class NullPointerException extends RuntimeException {
     public NullPointerException(String s) {
         super(s);
     }
+
+    /**
+     * Returns the detail message string of this throwable.
+     *
+     * <p> If a non-null message was supplied in a constructor it is
+     * returned. Otherwise, an implementation specific message or
+     * {@code null} is returned.
+     *
+     * @implNote
+     * If no explicit message was passed to the constructor, and as
+     * long as certain internal information is available, a verbose
+     * description of the null reference is returned.
+     * The internal information is not available in deserialized
+     * NullPointerExceptions.
+     *
+     * @return the detail message string, which may be {@code null}.
+     */
+    public String getMessage() {
+        String message = super.getMessage();
+        if (message == null) {
+            return getExtendedNPEMessage();
+        }
+        return message;
+    }
+
+    /**
+     * Get an extended exception message. This returns a string describing
+     * the location and cause of the exception. It returns null for
+     * exceptions where this is not applicable.
+     */
+    private native String getExtendedNPEMessage();
 }

src/java.base/share/native/libjava/NullPointerException.c

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2019 SAP SE. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+#include "jni.h"
+#include "jvm.h"
+
+#include "java_lang_NullPointerException.h"
+
+JNIEXPORT jstring JNICALL
+Java_java_lang_NullPointerException_getExtendedNPEMessage(JNIEnv *env, jobject throwable)
+{
+    return JVM_GetExtendedNPEMessage(env, throwable);
+}