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TBEL is a powerful expression language for ThingsBoard platform user-defined functions. Original implementation is based on MVEL.
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/**
 * MVEL 2.0
 * Copyright (C) 2007 The Codehaus
 * Mike Brock, Dhanji Prasanna, John Graham, Mark Proctor
 *
 * 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 org.mvel2.util;

import java.io.UnsupportedEncodingException;
import java.nio.charset.Charset;

import static java.lang.System.arraycopy;

/**
 *
 * @author Mike Brock
 */
public class StringAppender implements CharSequence {
  private static final int DEFAULT_SIZE = 15;

  private char[] str;
  private int capacity;
  private int size = 0;
  private byte[] btr;
  private String encoding;

  public StringAppender() {
    str = new char[capacity = DEFAULT_SIZE];
  }

  public StringAppender(final int capacity) {
    str = new char[this.capacity = capacity];
  }

  public StringAppender(final int capacity, final String encoding) {
    str = new char[this.capacity = capacity];
    this.encoding = encoding;
  }

  public StringAppender(final char c) {
    (str = new char[this.capacity = DEFAULT_SIZE])[0] = c;
  }

  public StringAppender(final char[] s) {
    capacity = size = (str = s).length;
  }

  public StringAppender(final CharSequence s) {
    str = new char[this.capacity = size = s.length()];
    for (int i = 0; i < str.length; i++)
      str[i] = s.charAt(i);

  }

  public StringAppender(final String s) {
    capacity = size = (str = s.toCharArray()).length;
  }

  public StringAppender append(final char[] chars) {
    if (chars.length > (capacity - size)) grow(chars.length);
    for (int i = 0; i < chars.length; size++) {
      str[size] = chars[i++];
    }
    return this;
  }

  public StringAppender append(final byte[] chars) {
    if (chars.length > (capacity - size)) grow(chars.length);
    for (int i = 0; i < chars.length; size++) {
      str[size] = (char) chars[i++];
    }
    return this;
  }

  public StringAppender append(final char[] chars, final int start, final int length) {
    if (length > (capacity - size)) grow(length);
    int x = start + length;
    for (int i = start; i < x; i++) {
      str[size++] = chars[i];
    }
    return this;
  }

  public StringAppender append(final byte[] chars, final int start, final int length) {
    if (length > (capacity - size)) grow(length);
    int x = start + length;
    for (int i = start; i < x; i++) {
      str[size++] = (char) chars[i];
    }
    return this;
  }

  public StringAppender append(final Object o) {
    return append(String.valueOf(o));
  }

  public StringAppender append(final CharSequence s) {
    if (s.length() > (capacity - size)) grow(s.length());
    for (int i = 0; i < s.length(); size++) {
      str[size] = s.charAt(i++);
    }
    return this;
  }

  public StringAppender append(final String s) {
    if (s == null) return this;

    final int len = s.length();
    if (len > (capacity - size)) {
      grow(len);
    }

    s.getChars(0, len, str, size);
    size += len;

    return this;
  }

  public StringAppender append(final char c) {
    if (size >= capacity) grow(size);
    str[size++] = c;
    return this;
  }

  public StringAppender append(final byte b) {
    if (btr == null)
      btr = new byte[capacity = DEFAULT_SIZE];
    if (size >= capacity)
      growByte(size * 2);
    btr[size++] = b;
    return this;
  }


  public int length() {
    return size;
  }

  private void grow(final int s) {
    if (capacity == 0) capacity = DEFAULT_SIZE;
    final char[] newArray = new char[capacity += s * 2];
    arraycopy(str, 0, newArray, 0, size);
    str = newArray;
  }

  private void growByte(final int s) {
    final byte[] newByteArray = new byte[capacity += s];
    arraycopy(btr, 0, newByteArray, 0, size);
    btr = newByteArray;
  }

  public char[] getChars(final int start, final int count) {
    char[] chars = new char[count];
    arraycopy(str, start, chars, 0, count);
    return chars;
  }

  public char[] toChars() {
    if (btr != null) {
      if (encoding == null)
        encoding = System.getProperty("file.encoding");
      String s;
      try {
        s = new String(btr, encoding);
      }
      catch (UnsupportedEncodingException e) {
        s = new String(btr);
      }
      return s.toCharArray();
    }
    final char[] chars = new char[size];
    arraycopy(str, 0, chars, 0, size);
    return chars;
  }

  public String toString() {
    if (btr != null) {
      if (encoding == null)
        encoding = System.getProperty("file.encoding");
      String s;
      try {
        s = new String(btr, 0, size, encoding);
      }
      catch (UnsupportedEncodingException e) {
        s = new String(btr, 0, size);
      }
      return s;
    }
    if (size == capacity) return new String(str);
    else return new String(str, 0, size);
  }

  public void getChars(int start, int count, char[] target, int offset) {
    int delta = offset;
    for (int i = start; i < count; i++) {
      target[delta++] = str[i];
    }
  }

  public void reset() {
    size = 0;
  }

  public char charAt(int index) {
    return str[index];
  }

  public CharSequence substring(int start, int end) {
    return new String(str, start, (end - start));
  }

  public CharSequence subSequence(int start, int end) {
    return substring(start, end);
  }
}