8315702: jcmd Thread.dump_to_file slow with millions of virtual threads

Reviewed-by: mchung, amenkov, jpai

src/java.base/share/classes/jdk/internal/vm/ThreadDumper.java

@@ -24,6 +24,7 @@
  */
 package jdk.internal.vm;
 
+import java.io.BufferedOutputStream;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.OutputStream;
@@ -110,7 +111,8 @@ private static byte[] dumpThreadsToFile(String file, boolean okayToOverwrite, bo
                 : new OpenOption[] { StandardOpenOption.CREATE_NEW };
         String reply;
         try (OutputStream out = Files.newOutputStream(path, options);
-             PrintStream ps = new PrintStream(out, true, StandardCharsets.UTF_8)) {
+             BufferedOutputStream bos = new BufferedOutputStream(out);
+             PrintStream ps = new PrintStream(bos, false, StandardCharsets.UTF_8)) {
             if (json) {
                 dumpThreadsToJson(ps);
             } else {
@@ -132,11 +134,12 @@ private static byte[] dumpThreadsToFile(String file, boolean okayToOverwrite, bo
      * This method is invoked by HotSpotDiagnosticMXBean.dumpThreads.
      */
     public static void dumpThreads(OutputStream out) {
-        PrintStream ps = new PrintStream(out, true, StandardCharsets.UTF_8);
+        BufferedOutputStream bos = new BufferedOutputStream(out);
+        PrintStream ps = new PrintStream(bos, false, StandardCharsets.UTF_8);
         try {
             dumpThreads(ps);
         } finally {
-            ps.flush();
+            ps.flush();  // flushes underlying stream
         }
     }
 
@@ -158,9 +161,10 @@ private static void dumpThreads(ThreadContainer container, PrintStream ps) {
 
     private static void dumpThread(Thread thread, PrintStream ps) {
         String suffix = thread.isVirtual() ? " virtual" : "";
-        ps.format("#%d \"%s\"%s%n", thread.threadId(), thread.getName(), suffix);
+        ps.println("#" + thread.threadId() + " \"" + thread.getName() + "\"" + suffix);
         for (StackTraceElement ste : thread.getStackTrace()) {
-            ps.format("      %s%n", ste);
+            ps.print("      ");
+            ps.println(ste);
         }
         ps.println();
     }
@@ -171,11 +175,12 @@ private static void dumpThread(Thread thread, PrintStream ps) {
      * This method is invoked by HotSpotDiagnosticMXBean.dumpThreads.
      */
     public static void dumpThreadsToJson(OutputStream out) {
-        PrintStream ps = new PrintStream(out, true, StandardCharsets.UTF_8);
+        BufferedOutputStream bos = new BufferedOutputStream(out);
+        PrintStream ps = new PrintStream(bos, false, StandardCharsets.UTF_8);
         try {
             dumpThreadsToJson(ps);
         } finally {
-            ps.flush();
+            ps.flush();  // flushes underlying stream
         }
     }
 
@@ -257,13 +262,16 @@ private static void dumpThreadsToJson(ThreadContainer container,
      */
     private static void dumpThreadToJson(Thread thread, PrintStream out, boolean more) {
         out.println("         {");
-        out.format("           \"tid\": \"%d\",%n", thread.threadId());
-        out.format("           \"name\": \"%s\",%n", escape(thread.getName()));
-        out.format("           \"stack\": [%n");
+        out.println("           \"tid\": \"" + thread.threadId() + "\",");
+        out.println("           \"name\": \"" + escape(thread.getName()) + "\",");
+        out.println("           \"stack\": [");
+
         int i = 0;
         StackTraceElement[] stackTrace = thread.getStackTrace();
         while (i < stackTrace.length) {
-            out.format("              \"%s\"", escape(stackTrace[i].toString()));
+            out.print("              \"");
+            out.print(escape(stackTrace[i].toString()));
+            out.print("\"");
             i++;
             if (i < stackTrace.length) {
                 out.println(",");