8325945: Error reporting should limit the number of String characters printed

Reviewed-by: thartmann, stuefe

src/hotspot/share/classfile/javaClasses.cpp

@@ -751,8 +751,16 @@ bool java_lang_String::equals(oop str1, oop str2) {
   return value_equals(value1, value2);
 }
 
-void java_lang_String::print(oop java_string, outputStream* st) {
+// Print the given string to the given outputStream, limiting the output to
+// at most max_length of the string's characters. If the length exceeds the
+// limit we print an abridged version of the string with the "middle" elided
+// and replaced by " ... (N characters ommitted) ... ". If max_length is odd
+// it is treated as max_length-1.
+void java_lang_String::print(oop java_string, outputStream* st, int max_length) {
   assert(java_string->klass() == vmClasses::String_klass(), "must be java_string");
+  // We need at least two characters to print A ... B
+  assert(max_length > 1, "invalid max_length: %d", max_length);
+
   typeArrayOop value  = java_lang_String::value_no_keepalive(java_string);
 
   if (value == nullptr) {
@@ -765,8 +773,17 @@ void java_lang_String::print(oop java_string, outputStream* st) {
   int length = java_lang_String::length(java_string, value);
   bool is_latin1 = java_lang_String::is_latin1(java_string);
 
+  bool abridge = length > max_length;
+
   st->print("\"");
   for (int index = 0; index < length; index++) {
+    // If we need to abridge and we've printed half the allowed characters
+    // then jump to the tail of the string.
+    if (abridge && index >= max_length / 2) {
+      st->print(" ... (%d characters ommitted) ... ", length - 2 * (max_length / 2));
+      index = length - (max_length / 2);
+      abridge = false; // only do this once
+    }
     jchar c = (!is_latin1) ?  value->char_at(index) :
                              ((jchar) value->byte_at(index)) & 0xff;
     if (c < ' ') {
@@ -776,6 +793,10 @@ void java_lang_String::print(oop java_string, outputStream* st) {
     }
   }
   st->print("\"");
+
+  if (length > max_length) {
+    st->print(" (abridged) ");
+  }
 }
 
 // java_lang_Class

src/hotspot/share/classfile/javaClasses.hpp

@@ -194,7 +194,7 @@ class java_lang_String : AllStatic {
   static inline bool is_instance(oop obj);
 
   // Debugging
-  static void print(oop java_string, outputStream* st);
+  static void print(oop java_string, outputStream* st, int max_length = MaxStringPrintSize);
   friend class JavaClasses;
   friend class StringTable;
 };

src/hotspot/share/prims/whitebox.cpp

@@ -179,6 +179,14 @@ WB_ENTRY(jlong, WB_GetVMLargePageSize(JNIEnv* env, jobject o))
   return os::large_page_size();
 WB_END
 
+WB_ENTRY(jstring, WB_PrintString(JNIEnv* env, jobject wb, jstring str, jint max_length))
+  ResourceMark rm(THREAD);
+  stringStream sb;
+  java_lang_String::print(JNIHandles::resolve(str), &sb, max_length);
+  oop result = java_lang_String::create_oop_from_str(sb.as_string(), THREAD);
+  return (jstring) JNIHandles::make_local(THREAD, result);
+WB_END
+
 class WBIsKlassAliveClosure : public LockedClassesDo {
     Symbol* _name;
     int _count;
@@ -2959,6 +2967,7 @@ static JNINativeMethod methods[] = {
   {CC"preTouchMemory",  CC"(JJ)V",                    (void*)&WB_PreTouchMemory},
   {CC"cleanMetaspaces", CC"()V",                      (void*)&WB_CleanMetaspaces},
   {CC"rss", CC"()J",                                  (void*)&WB_Rss},
+  {CC"printString", CC"(Ljava/lang/String;I)Ljava/lang/String;", (void*)&WB_PrintString},
 };
 
 

src/hotspot/share/runtime/globals.hpp

@@ -1304,6 +1304,12 @@ const int ObjectAlignmentInBytes = 8;
   develop(int, MaxElementPrintSize, 256,                                    \
           "maximum number of elements to print")                            \
                                                                             \
+  develop(int, MaxStringPrintSize, 256,                                     \
+          "maximum number of characters to print for a java.lang.String "   \
+          "in the VM. If exceeded, an abridged version of the string is "   \
+          "printed with the middle of the string elided.")                  \
+          range(2, O_BUFLEN)                                                \
+                                                                            \
   develop(intx, MaxSubklassPrintSize, 4,                                    \
           "maximum number of subklasses to print when printing klass")      \
                                                                             \

test/hotspot/jtreg/runtime/PrintingTests/StringPrinting.java

@@ -0,0 +1,122 @@
+/*
+ * Copyright (c) 2024, Oracle and/or its affiliates. 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.
+ *
+ * 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.
+ */
+
+/**
+ * @test
+ * @bug 8325945
+ * @summary Test abridged VM String printing
+ * @library /test/lib
+ * @build jdk.test.whitebox.WhiteBox
+ * @run driver jdk.test.lib.helpers.ClassFileInstaller jdk.test.whitebox.WhiteBox
+ * @run main/othervm -Xbootclasspath/a:. -XX:+WhiteBoxAPI StringPrinting
+ */
+
+import jdk.test.whitebox.WhiteBox;
+
+public class StringPrinting {
+
+    private static final WhiteBox WB = WhiteBox.getWhiteBox();
+
+    static void checkEqual(String s1, String s2) {
+        if (!s1.equals(s2)) {
+            throw new RuntimeException("Different strings: " + s1 + " vs " + s2);
+        }
+    }
+
+    static void checkEqual(int len1, int len2) {
+        if (len1 != len2) {
+            throw new RuntimeException("Different lengths: " + len1 + " vs " + len2);
+        }
+    }
+
+    public static void main(String[] args) {
+        // Modified string format is "xxx ... (N characters ommitted) ... xxx" (abridged)
+        final String elipse = " ... ";
+        final String text = " characters ommitted)";
+        final String abridged = "\" (abridged) ";
+
+        // Define a set of maxLengths for ease of inspection in the test outout
+        // Note: maxLength must be >= 2
+        int[] maxLengths = new int[] { 2, 3, 16, 256, 512 };
+        for (int maxLength : maxLengths) {
+            // Test string lengths around maxLength and "much" bigger
+            // than maxLength
+            int[] strLengths = new int[] { maxLength - 1,
+                                           maxLength,
+                                           maxLength + 1,
+                                           2 * maxLength
+            };
+            for (int length : strLengths) {
+                System.out.println("Testing string length " + length + " with maxLength " + maxLength);
+                String s = "x".repeat(length);
+                String r = WB.printString(s, maxLength);
+                if (length <= maxLength) {
+                    // Strip off the double-quotes that surround the string
+                    if (r.charAt(0) == '\"' && r.charAt(r.length() - 1) == '\"') {
+                        r = r.substring(1, r.length() - 1);
+                    } else {
+                        throw new RuntimeException("String was not quoted as expected: " + r);
+                    }
+                    checkEqual(s, r);
+                } else {
+                    // Strip off leading double-quote
+                    if (r.charAt(0) == '\"') {
+                        r = r.substring(1, r.length());
+                    } else {
+                        throw new RuntimeException("String was not quoted as expected: " + r);
+                    }
+
+                    // Strip off abridged
+                    if (r.endsWith(abridged)) {
+                        r = r.substring(0, r.length() - abridged.length());
+                    } else {
+                        throw new RuntimeException("String was not marked abridged as expected: " + r);
+                    }
+
+                    // Now extract the two "halves"
+                    int elipseStart = r.indexOf(elipse);
+                    String firstHalf = r.substring(0, elipseStart);
+                    int secondElipseStart = r.lastIndexOf(elipse);
+                    String secondHalf = r.substring(secondElipseStart + elipse.length(), r.length());
+
+                    System.out.println("S1: >" + firstHalf + "<");
+                    System.out.println("S2: >" + secondHalf + "<");
+
+                    checkEqual(firstHalf.length(), maxLength / 2);
+                    checkEqual(secondHalf.length(), maxLength /2);
+
+                    // Now check number of characters ommitted
+                    String tail = r.substring(r.indexOf("("), r.length());
+                    int numberEnd = tail.indexOf(" ");
+                    String nChars = tail.substring(1, numberEnd);
+                    System.out.println("N: >" + nChars + "<");
+
+                    // Now add all the bits back together to get the expected full length
+                    int fullLength = maxLength / 2 + elipse.length() + 1 /* for ( */
+                        + nChars.length() + text.length() + elipse.length() + maxLength / 2;
+                    checkEqual(r.length(), fullLength);
+                }
+            }
+        }
+    }
+}

test/lib/jdk/test/whitebox/WhiteBox.java

@@ -99,8 +99,13 @@ public class WhiteBox {
   }
 
   // Runtime
-  // Make sure class name is in the correct format
+
+  // Returns the potentially abridged form of `str` as it would be
+  // printed by the VM.
+  public native String printString(String str, int maxLength);
+
   public int countAliveClasses(String name) {
+    // Make sure class name is in the correct format
     return countAliveClasses0(name.replace('.', '/'));
   }
   private native int countAliveClasses0(String name);