code: /*
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You 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.apache.skywalking.oap.server.core.alarm.provider.expression;
  
  import lombok.extern.slf4j.Slf4j;
  import org.mvel2.MVEL;
  import org.mvel2.ParserContext;
  
  import java.util.Map;
  import java.util.Set;
  import java.util.concurrent.ConcurrentHashMap;
  
  /**
   * Expression support eval java basic expressions, just like groovy script
   * The internal detail is it first compile the expression to a parseTree then execute the parseTree with data
   * It caches the compiled expression for sake of performance
   */
  @Slf4j
  public class Expression {
      private final Map<String, Object> expressionCache;
      private final ExpressionContext context;
  
      public Expression(ExpressionContext context) {
          this.context = context;
          this.expressionCache = new ConcurrentHashMap<>();
      }
  
      /**
       * Eval the given expression using empty data context
       */
      public Object eval(String expression) {
          return eval(expression, null);
      }
  
      /**
       * Eval the given expression with data context
       */
      public Object eval(String expression, Map<String, Object> vars) {
          try {
              Object obj = compile(expression, context);
              return MVEL.executeExpression(obj, vars);
          } catch (Throwable e) {
              log.error("eval expression {} error", expression, e);
              return null;
          }
      }
  
      /**
       * Compile the given expression to a parseTree
       */
      public Object compile(String expression, ExpressionContext pctx) {
          return expressionCache.computeIfAbsent(expression, s -> MVEL.compileExpression(expression, pctx.getContext()));
      }
  
      /**
       * Analysis expression dependencies
       */
      public Set<String> analysisInputs(String expression) {
          ParserContext pCtx = ParserContext.create();
          MVEL.analysisCompile(expression, pCtx);
          Map<String, Class> inputsMap = pCtx.getInputs();
          return inputsMap.keySet();
      }
  }
  