--- a/src/hotspot/share/utilities/ostream.cpp	Wed May 22 07:10:54 2019 +0200
+++ b/src/hotspot/share/utilities/ostream.cpp	Wed May 22 09:33:22 2019 +0200
@@ -309,10 +309,9 @@
 
 stringStream::stringStream(size_t initial_size) : outputStream() {
   buffer_length = initial_size;
-  buffer        = NEW_RESOURCE_ARRAY(char, buffer_length);
+  buffer        = NEW_C_HEAP_ARRAY(char, buffer_length, mtInternal);
   buffer_pos    = 0;
   buffer_fixed  = false;
-  DEBUG_ONLY(rm = Thread::current()->current_resource_mark();)
 }
 
 // useful for output to fixed chunks of memory, such as performance counters
@@ -337,15 +336,7 @@
       if (end < buffer_length * 2) {
         end = buffer_length * 2;
       }
-      char* oldbuf = buffer;
-      assert(rm == NULL || Thread::current()->current_resource_mark() == rm,
-             "StringStream is re-allocated with a different ResourceMark. Current: "
-             PTR_FORMAT " original: " PTR_FORMAT,
-             p2i(Thread::current()->current_resource_mark()), p2i(rm));
-      buffer = NEW_RESOURCE_ARRAY(char, end);
-      if (buffer_pos > 0) {
-        memcpy(buffer, oldbuf, buffer_pos);
-      }
+      buffer = REALLOC_C_HEAP_ARRAY(char, buffer, end, mtInternal);
       buffer_length = end;
     }
   }
@@ -370,7 +361,11 @@
   return copy;
 }
 
-stringStream::~stringStream() {}
+stringStream::~stringStream() {
+  if (buffer_fixed == false && buffer != NULL) {
+    FREE_C_HEAP_ARRAY(char, buffer);
+  }
+}
 
 xmlStream*   xtty;
 outputStream* tty;

--- a/src/hotspot/share/utilities/ostream.hpp	Wed May 22 07:10:54 2019 +0200
+++ b/src/hotspot/share/utilities/ostream.hpp	Wed May 22 09:33:22 2019 +0200
@@ -190,19 +190,25 @@
   }
 };
 
-// for writing to strings; buffer will expand automatically
+// for writing to strings; buffer will expand automatically.
+// Buffer will always be zero-terminated.
 class stringStream : public outputStream {
  protected:
   char*  buffer;
   size_t buffer_pos;
   size_t buffer_length;
   bool   buffer_fixed;
-  DEBUG_ONLY(ResourceMark* rm;)
  public:
+  // Create a stringStream using an internal buffer of initially initial_bufsize size;
+  // will be enlarged on demand. There is no maximum cap.
   stringStream(size_t initial_bufsize = 256);
+  // Creates a stringStream using a caller-provided buffer. Will truncate silently if
+  // it overflows.
   stringStream(char* fixed_buffer, size_t fixed_buffer_size);
   ~stringStream();
   virtual void write(const char* c, size_t len);
+  // Return number of characters written into buffer, excluding terminating zero and
+  // subject to truncation in static buffer mode.
   size_t      size() { return buffer_pos; }
   const char* base() { return buffer; }
   void  reset() { buffer_pos = 0; _precount = 0; _position = 0; }

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/hotspot/gtest/utilities/test_ostream.cpp	Wed May 22 09:33:22 2019 +0200
@@ -0,0 +1,69 @@
+/*
+ * 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.
+ *
+ * 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 "precompiled.hpp"
+#include "memory/resourceArea.hpp"
+#include "runtime/os.hpp"
+#include "utilities/globalDefinitions.hpp"
+#include "utilities/ostream.hpp"
+
+#include "unittest.hpp"
+
+static size_t print_lorem(outputStream* st, bool short_len) {
+  // Create a ResourceMark just to make sure the stream does not use ResourceArea
+  ResourceMark rm;
+  static const char* const lorem = "Lorem ipsum dolor sit amet, consectetur adipiscing elit, "
+      "sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Lacinia at quis "
+      "risus sed vulputate odio ut enim blandit. Amet risus nullam eget felis eget. Viverra "
+      "orci sagittis eu volutpat odio facilisis mauris sit. Erat velit scelerisque in dictum non.";
+  static const size_t len_lorem = strlen(lorem);
+  size_t len;
+  if (short_len) {
+    len = os::random() % 10;
+  } else {
+    len = MAX2(1, (int)(os::random() % len_lorem));
+  }
+  st->write(lorem, len);
+  return len;
+}
+
+static void do_test_stringStream_dynamic_realloc(bool short_len) {
+  stringStream ss(2); // small buffer to force lots of reallocations.
+  size_t written = 0;
+  for (int i = 0; i < 1000; i ++) {
+    written += print_lorem(&ss, short_len);
+    ASSERT_EQ(ss.size(), written);
+    // Internal buffer should always be zero-terminated.
+    ASSERT_EQ(ss.base()[ss.size()], '\0');
+  }
+}
+
+TEST_VM(ostream, stringStream_dynamic_realloc_1) {
+  do_test_stringStream_dynamic_realloc(false);
+}
+
+TEST_VM(ostream, stringStream_dynamic_realloc_2) {
+  do_test_stringStream_dynamic_realloc(true);
+}