8358533: Improve performance of java.io.Reader.readAllLines

src/java.base/share/classes/java/io/Reader.java

@@ -27,6 +27,7 @@
 
 import java.nio.CharBuffer;
 import java.nio.ReadOnlyBufferException;
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Objects;
 
@@ -397,16 +398,6 @@ public int read(char[] cbuf) throws IOException {
      */
     public abstract int read(char[] cbuf, int off, int len) throws IOException;
 
-    private String readAllCharsAsString() throws IOException {
-        StringBuilder result = new StringBuilder();
-        char[] cbuf = new char[TRANSFER_BUFFER_SIZE];
-        int nread;
-        while ((nread = read(cbuf, 0, cbuf.length)) != -1) {
-            result.append(cbuf, 0, nread);
-        }
-        return result.toString();
-    }
-
     /**
      * Reads all remaining characters as lines of text. This method blocks until
      * all remaining characters have been read and end of stream is detected,
@@ -457,7 +448,64 @@ private String readAllCharsAsString() throws IOException {
      * @since 25
      */
     public List<String> readAllLines() throws IOException {
-        return readAllCharsAsString().lines().toList();
+        char[] cb = new char[TRANSFER_BUFFER_SIZE];
+        int pos = 0;
+        List<String> lines = new ArrayList<String>();
+
+        StringBuilder sb = new StringBuilder(82);
+        int n = read(cb, 0, cb.length);
+        boolean eos = (n == -1);
+        while (!eos) {
+            boolean eol = false;
+            boolean stringAdded = false;
+            while (!eol) {
+                // Find the next line terminator. If none is found,
+                // "term" will equal "n".
+                int term = pos;
+                while (term < n) {
+                    char c = cb[term];
+                    if (c == '\r' || c == '\n')
+                        break;
+                    term++;
+                }
+
+                // Terminator found so at EOL.
+                if (term < n)
+                    eol = true;
+
+                if (term == pos) {
+                    // Current position is terminator so skip it.
+                    pos++;
+                } else { // term > pos
+                    if (eol && sb.length() == 0) {
+                        // Create and add a string to avoid the StringBuilder.
+                        lines.add(new String(cb, pos, term - pos));
+                        stringAdded = true;
+                    } else
+                        sb.append(cb, pos, term - pos);
+                    pos = term + 1;
+                }
+
+                if (pos >= n) {
+                    // Buffer content consumed so reload it.
+                    if ((n = read(cb, 0, cb.length)) < 0) {
+                        eos = eol = true;
+                        break;
+                    }
+                    pos = 0;
+                }
+            }
+
+            if (!stringAdded) {
+                // Derive a string and add it to the list.
+                lines.add(sb.toString());
+                sb.setLength(0);
+            }
+
+            eol = false;
+        }
+
+        return lines;
     }
 
     /**
@@ -499,7 +547,13 @@ public List<String> readAllLines() throws IOException {
      * @since 25
      */
     public String readAllAsString() throws IOException {
-        return readAllCharsAsString();
+        StringBuilder result = new StringBuilder();
+        char[] cbuf = new char[TRANSFER_BUFFER_SIZE];
+        int nread;
+        while ((nread = read(cbuf, 0, cbuf.length)) != -1) {
+            result.append(cbuf, 0, nread);
+        }
+        return result.toString();
     }
 
     /** Maximum skip-buffer size */

test/micro/org/openjdk/bench/java/io/ReaderReadAllLines.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 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
+ * 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.
+ */
+package org.openjdk.bench.java.io;
+
+import java.io.CharArrayReader;
+import java.io.IOException;
+import java.io.Reader;
+import java.util.List;
+import java.util.Random;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.Setup;
+import org.openjdk.jmh.annotations.State;
+
+@State(Scope.Benchmark)
+public class ReaderReadAllLines {
+
+    private char[] chars = null;
+
+    @Setup
+    public void setup() throws IOException {
+        final int len = 128_000;
+        chars = new char[len];
+        Random rnd = new Random(System.nanoTime());
+        int off = 0;
+        while (off < len) {
+            int lineLen = 40 + rnd.nextInt(40);
+            if (lineLen > len - off) {
+                off = len;
+            } else {
+                chars[off + lineLen] = '\n';
+                off += lineLen;
+            }
+        }
+    }
+
+    @Benchmark
+    public List<String> readAllLines() throws IOException {
+        List<String> lines;
+        try (Reader reader = new CharArrayReader(chars);) {
+            lines = reader.readAllLines();
+        }
+        return lines;
+    }
+}