Java: Pulling in protobuf's faster UTF-8 encoder. (#5035)

* Pulling in protobuf's faster UTF-8 encoder.

* Remove Utf8 unsafe code.

java/com/google/flatbuffers/Utf8Old.java

@@ -0,0 +1,99 @@
+/*
+ * Copyright 2014 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.flatbuffers;
+
+import java.nio.ByteBuffer;
+import java.nio.CharBuffer;
+import java.nio.charset.CharacterCodingException;
+import java.nio.charset.CharsetDecoder;
+import java.nio.charset.CharsetEncoder;
+import java.nio.charset.CoderResult;
+import java.nio.charset.StandardCharsets;
+
+/**
+ * This class implements the Utf8 API using the Java Utf8 encoder. Use
+ * Utf8.setDefault(new Utf8Old()); to use it.
+ */
+public class Utf8Old extends Utf8 {
+
+  private static class Cache {
+    final CharsetEncoder encoder;
+    final CharsetDecoder decoder;
+    CharSequence lastInput = null;
+    ByteBuffer lastOutput = null;
+
+    Cache() {
+      encoder = StandardCharsets.UTF_8.newEncoder();
+      decoder = StandardCharsets.UTF_8.newDecoder();
+    }
+  }
+
+  private static final ThreadLocal<Cache> CACHE =
+      ThreadLocal.withInitial(() -> new Cache());
+
+  // Play some games so that the old encoder doesn't pay twice for computing
+  // the length of the encoded string.
+
+  @Override
+  public int encodedLength(CharSequence in) {
+    final Cache cache = CACHE.get();
+    int estimated = (int) (in.length() * cache.encoder.maxBytesPerChar());
+    if (cache.lastOutput == null || cache.lastOutput.capacity() < estimated) {
+      cache.lastOutput = ByteBuffer.allocate(Math.max(128, estimated));
+    }
+    cache.lastOutput.clear();
+    cache.lastInput = in;
+    CharBuffer wrap = (in instanceof CharBuffer) ?
+                          (CharBuffer) in : CharBuffer.wrap(in);
+    CoderResult result = cache.encoder.encode(wrap, cache.lastOutput, true);
+    if (result.isError()) {
+      try {
+        result.throwException();
+      } catch (CharacterCodingException e) {
+        throw new IllegalArgumentException("bad character encoding", e);
+      }
+    }
+    return cache.lastOutput.remaining();
+  }
+
+  @Override
+  public void encodeUtf8(CharSequence in, ByteBuffer out) {
+    final Cache cache = CACHE.get();
+    if (cache.lastInput != in) {
+      // Update the lastOutput to match our input, although flatbuffer should
+      // never take this branch.
+      encodedLength(in);
+    }
+    out.put(cache.lastOutput);
+  }
+
+  @Override
+  public String decodeUtf8(ByteBuffer buffer, int offset, int length) {
+    CharsetDecoder decoder = CACHE.get().decoder;
+    decoder.reset();
+    buffer = buffer.duplicate();
+    buffer.position(offset);
+    buffer.limit(offset + length);
+    try {
+      CharBuffer result = decoder.decode(buffer);
+      result.flip();
+      return result.toString();
+    } catch (CharacterCodingException e) {
+      throw new IllegalArgumentException("Bad encoding", e);
+    }
+  }
+}