8355636: Reuse StringBuilder in printStackTrace

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

@@ -365,8 +365,16 @@ public String toString() {
                 + methodName.length() + 1
                 + Math.max(UNKNOWN_SOURCE.length(), length(fileName)) + 1
                 + 12;
+        return appendTo(
+                new StringBuilder(estimatedLength))
+                .toString();
+    }
 
-        StringBuilder sb = new StringBuilder(estimatedLength);
+    /**
+     * Prints the toString result to the given buf, avoiding extra string allocations.
+     */
+    StringBuilder appendTo(StringBuilder sb) {
+        int startingLength = sb.length();
         if (!dropClassLoaderName() && classLoaderName != null && !classLoaderName.isEmpty()) {
             sb.append(classLoaderName).append('/');
         }
@@ -378,7 +386,7 @@ public String toString() {
             }
         }
 
-        if (sb.length() > 0) {
+        if (sb.length() != startingLength) {
             sb.append('/');
         }
 
@@ -393,9 +401,7 @@ public String toString() {
                 sb.append(':').append(lineNumber);
             }
         }
-        sb.append(')');
-
-        return sb.toString();
+        return sb.append(')');
     }
 
     private static int length(String s) {

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1994, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1994, 2025, 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
@@ -687,26 +687,42 @@ public void printStackTrace(PrintStream s) {
     }
 
     private void printStackTrace(PrintStreamOrWriter s) {
-        // Guard against malicious overrides of Throwable.equals by
-        // using a Set with identity equality semantics.
-        Set<Throwable> dejaVu = Collections.newSetFromMap(new IdentityHashMap<>());
-        dejaVu.add(this);
-
         synchronized(s.lock()) {
             // Print our stack trace
             s.println(this);
+            var lineBuffer = new StringBuilder(128); // arbitrary size
+            lineBuffer.append("\tat ");
+            int prefixLen = lineBuffer.length();
+
             StackTraceElement[] trace = getOurStackTrace();
-            for (StackTraceElement traceElement : trace)
-                s.println("\tat " + traceElement);
+            for (int i = 0; i < trace.length; i++) {
+                if (i != 0) {
+                    lineBuffer.setLength(prefixLen);
+                }
+                trace[i].appendTo(lineBuffer);
+                s.println(lineBuffer.toString());
+            }
 
-            // Print suppressed exceptions, if any
-            for (Throwable se : getSuppressed())
-                se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION, "\t", dejaVu);
+            var suppressed = getSuppressed();
+            var ourCause = getCause();
 
-            // Print cause, if any
-            Throwable ourCause = getCause();
-            if (ourCause != null)
-                ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, "", dejaVu);
+            if (suppressed.length > 0 || ourCause != null) {
+                /*
+                 * Guard against malicious overrides of Throwable.equals by
+                 * using a Set with identity equality semantics.
+                 */
+                Set<Throwable> dejaVu = Collections.newSetFromMap(new IdentityHashMap<>());
+                dejaVu.add(this);
+
+                // Print suppressed exceptions, if any
+                for (Throwable se : suppressed)
+                    se.printEnclosedStackTrace(s, lineBuffer, trace, SUPPRESSED_CAPTION, "\t", dejaVu);
+
+                // Print cause, if any
+                if (ourCause != null) {
+                    ourCause.printEnclosedStackTrace(s, lineBuffer, trace, CAUSE_CAPTION, "", dejaVu);
+                }
+            }
         }
     }
 
@@ -715,13 +731,16 @@ private void printStackTrace(PrintStreamOrWriter s) {
      * stack trace.
      */
     private void printEnclosedStackTrace(PrintStreamOrWriter s,
+                                         StringBuilder lineBuffer,
                                          StackTraceElement[] enclosingTrace,
                                          String caption,
                                          String prefix,
                                          Set<Throwable> dejaVu) {
         assert Thread.holdsLock(s.lock());
+        lineBuffer.setLength(0);
         if (dejaVu.contains(this)) {
-            s.println(prefix + caption + "[CIRCULAR REFERENCE: " + this + "]");
+            s.println(
+                    lineBuffer.append(prefix).append(caption).append("[CIRCULAR REFERENCE: ").append(this).append("]"));
         } else {
             dejaVu.add(this);
             // Compute number of frames in common between this and enclosing trace
@@ -734,21 +753,33 @@ private void printEnclosedStackTrace(PrintStreamOrWriter s,
             int framesInCommon = trace.length - 1 - m;
 
             // Print our stack trace
-            s.println(prefix + caption + this);
-            for (int i = 0; i <= m; i++)
-                s.println(prefix + "\tat " + trace[i]);
-            if (framesInCommon != 0)
-                s.println(prefix + "\t... " + framesInCommon + " more");
+            s.println(
+                    lineBuffer.append(prefix).append(caption).append(this));
+            lineBuffer.setLength(prefix.length());
+            lineBuffer.append("\tat ");
+            int indent = lineBuffer.length();
+            for (int i = 0; i <= m; i++) {
+                if (i != 0) {
+                    lineBuffer.setLength(indent);
+                }
+                s.println(
+                        trace[i].appendTo(lineBuffer));
+            }
+            if (framesInCommon != 0) {
+                lineBuffer.setLength(prefix.length());
+                s.println(
+                        lineBuffer.append("\t... ").append(framesInCommon).append(" more"));
+            }
 
             // Print suppressed exceptions, if any
             for (Throwable se : getSuppressed())
-                se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION,
-                                           prefix +"\t", dejaVu);
+                se.printEnclosedStackTrace(s, lineBuffer, trace, SUPPRESSED_CAPTION,
+                        prefix.concat("\t"), dejaVu);
 
             // Print cause, if any
             Throwable ourCause = getCause();
             if (ourCause != null)
-                ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, prefix, dejaVu);
+                ourCause.printEnclosedStackTrace(s, lineBuffer, trace, CAUSE_CAPTION, prefix, dejaVu);
         }
     }