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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;

import org.mvel2.ast.Proto;
import org.mvel2.compiler.AbstractParser;
import org.mvel2.integration.Interceptor;
import org.mvel2.integration.VariableResolverFactory;
import org.mvel2.integration.impl.ClassImportResolverFactory;
import org.mvel2.integration.impl.StackResetResolverFactory;
import org.mvel2.util.MethodStub;

import java.io.Serializable;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.util.Collections;
import java.util.EnumSet;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

import static org.mvel2.util.ParseTools.forNameWithInner;

/**
 * The resusable parser configuration object.
 */
public class ParserConfiguration implements Serializable {

  protected final Map imports = new ConcurrentHashMap();
  protected final Map> nonConvertableMethods = new ConcurrentHashMap<>();
  protected HashSet packageImports;
  protected Map interceptors;
  protected transient ClassLoader classLoader;

  private final transient Set nonValidImports = Collections.newSetFromMap( new ConcurrentHashMap() );

  private boolean allowNakedMethCall = MVEL.COMPILER_OPT_ALLOW_NAKED_METH_CALL;

  private boolean allowBootstrapBypass = true;

  private VariableResolverFactory threadUnsafeVariableResolverFactory;

  public ParserConfiguration() {
  }

  public ParserConfiguration(Map imports, Map interceptors) {
    addAllImports(imports);
    this.interceptors = interceptors;
  }

  public ParserConfiguration(Map imports, HashSet packageImports,
                             Map interceptors) {
    addAllImports(imports);
    this.packageImports = packageImports;
    this.interceptors = interceptors;
  }

  public HashSet getPackageImports() {
    return packageImports;
  }

  public void setPackageImports(HashSet packageImports) {
    this.packageImports = packageImports;
  }

  public Class getImport(String name) {
    if (imports.containsKey(name) && imports.get(name) instanceof Class) {
      return (Class) imports.get(name);
    }
    return (Class) (AbstractParser.LITERALS.get(name) instanceof Class ? AbstractParser.LITERALS.get(name) : null);
  }

  public MethodStub getStaticImport(String name) {
    return (MethodStub) imports.get(name);
  }

  public Object getStaticOrClassImport(String name) {
    return imports.containsKey(name) ? imports.get(name) : AbstractParser.LITERALS.get(name);
  }

  public void addPackageImport(String packageName) {
    if (packageImports == null) packageImports = new LinkedHashSet();
    packageImports.add(packageName);
    if (!addClassMemberStaticImports(packageName)) packageImports.add(packageName);
  }

  private boolean addClassMemberStaticImports(String packageName) {
    try {
      Class c = Class.forName(packageName);
      if (c.isEnum()) {

        //noinspection unchecked
        for (Enum e : (EnumSet) EnumSet.allOf(c)) {
          imports.put(e.name(), e);
        }
        return true;
      }
      else {
        for (Field f : c.getDeclaredFields()) {
          if ((f.getModifiers() & (Modifier.STATIC | Modifier.PUBLIC)) == (Modifier.STATIC | Modifier.PUBLIC)) {
            imports.put(f.getName(), f.get(null));
          }
        }

      }
    }
    catch (ClassNotFoundException e) {
      // do nothing.
    }
    catch (IllegalAccessException e) {
      throw new RuntimeException("error adding static imports for: " + packageName, e);
    }
    return false;
  }

  public void addAllImports(Map imports) {
    if (imports == null) return;

    Object o;

    for (Map.Entry entry : imports.entrySet()) {
      if ((o = entry.getValue()) instanceof Method) {
        this.imports.put(entry.getKey(), new MethodStub((Method) o));
      }
      else {
        this.imports.put(entry.getKey(), o);
      }
    }
  }

  private boolean checkForDynamicImport(String className) {
    if (packageImports == null) return false;
    if (!Character.isJavaIdentifierStart(className.charAt(0))) return false;
    if (nonValidImports.contains(className)) return false;

    int found = 0;
    Class cls = null;
    for (String pkg : packageImports) {
      try {
        cls = forNameWithInner( pkg + "." + className, getClassLoader() );
        found++;
      }
      catch (Throwable cnfe) {
        // do nothing.
      }
    }

    if (found > 1) throw new RuntimeException("ambiguous class name: " + className);
    if (found == 1) {
      addImport(className, cls);
      return true;
    }

    cacheNegativeHitForDynamicImport(className);
    return false;
  }

  public boolean hasImport(String name) {
    return (imports.containsKey(name)) ||
        AbstractParser.CLASS_LITERALS.containsKey(name) ||
        checkForDynamicImport(name);
  }
  public boolean isNonConvertableMethod(Class clazz, String method) {
    var methods = nonConvertableMethods.get(clazz.getName());
    return methods != null && methods.contains(method);
  }

  public void addImport(Class cls) {
    addImport(cls.getSimpleName(), cls);
  }

  public void addImport(String name, Class cls) {
    this.imports.put(name, cls);
  }
  public void registerNonConvertableMethods(Class clazz, Set methods) {
    this.nonConvertableMethods.put(clazz.getName(), methods);
  }

  public void addImport(String name, Proto proto) {
    this.imports.put(name, proto);
  }

  public void addImport(String name, Method method) {
    addImport(name, new MethodStub(method));
  }

  public void addImport(String name, MethodStub method) {
    this.imports.put(name, method);
  }

  public Map getInterceptors() {
    return interceptors;
  }

  public void setInterceptors(Map interceptors) {
    this.interceptors = interceptors;
  }

  public Map getImports() {
    return imports;
  }

  public void setImports(Map imports) {
    if (imports == null) return;

    Object val;

    for (Map.Entry entry : imports.entrySet()) {
      if ((val = entry.getValue()) instanceof Class) {
        addImport(entry.getKey(), (Class) val);
      }
      else if (val instanceof Method) {
        addImport(entry.getKey(), (Method) val);
      }
      else if (val instanceof MethodStub) {
        addImport(entry.getKey(), (MethodStub) val);
      }
      else if (val instanceof Proto) {
        addImport(entry.getKey(), (Proto) entry.getValue());
      }
      else {
        throw new RuntimeException("invalid element in imports map: " + entry.getKey() + " (" + val + ")");
      }
    }
  }

  public boolean hasImports() {
    return !imports.isEmpty() || (packageImports != null && packageImports.size() != 0);
  }

  public ClassLoader getClassLoader() {
    return classLoader == null ? classLoader = Thread.currentThread().getContextClassLoader() : classLoader;
  }

  public void setClassLoader(ClassLoader classLoader) {
    this.classLoader = classLoader;
  }

  public void setAllImports(Map imports) {
    this.imports.clear();
    if (imports != null) this.imports.putAll(imports);
  }

  public void setImports(HashMap imports) {
    // TODO: this method is here for backward compatibility. Could it be removed/deprecated?
    setAllImports(imports);
  }

  private void cacheNegativeHitForDynamicImport(String negativeHit) {
    nonValidImports.add(negativeHit);
  }

  public void flushCaches() {
    nonValidImports.clear();
  }

  public boolean isAllowNakedMethCall() {
    return allowNakedMethCall;
  }

  public void setAllowNakedMethCall(boolean allowNakedMethCall) {
    this.allowNakedMethCall = allowNakedMethCall;
  }

  public boolean isAllowBootstrapBypass() {
    return allowBootstrapBypass;
  }

  public void setAllowBootstrapBypass(boolean allowBootstrapBypass) {
    this.allowBootstrapBypass = allowBootstrapBypass;
  }

  public VariableResolverFactory getVariableFactory(VariableResolverFactory factory) {
    if (MVEL.RUNTIME_OPT_THREAD_UNSAFE) {
      if (threadUnsafeVariableResolverFactory == null) {
        threadUnsafeVariableResolverFactory = createVariableResolverFactory(factory);
      } else {
        if (threadUnsafeVariableResolverFactory instanceof StackResetResolverFactory) {
          ((StackResetResolverFactory) threadUnsafeVariableResolverFactory).setDelegate(factory);
        } else {
          threadUnsafeVariableResolverFactory.setNextFactory(factory);
        }
      }
      return threadUnsafeVariableResolverFactory;
    }

    return createVariableResolverFactory(factory);
  }

  private VariableResolverFactory createVariableResolverFactory(VariableResolverFactory factory) {
    return hasImports() ? new ClassImportResolverFactory(this, factory, true) : new StackResetResolverFactory(factory);
  }
}