/*
 * Copyright 2002-2020 the original author or authors.
 *
 * 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
 *
 *      https://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.springframework.expression.spel.standard;

import java.util.concurrent.atomic.AtomicInteger;

import org.springframework.core.convert.TypeDescriptor;
import org.springframework.expression.EvaluationContext;
import org.springframework.expression.EvaluationException;
import org.springframework.expression.Expression;
import org.springframework.expression.TypedValue;
import org.springframework.expression.common.ExpressionUtils;
import org.springframework.expression.spel.CompiledExpression;
import org.springframework.expression.spel.ExpressionState;
import org.springframework.expression.spel.SpelCompilerMode;
import org.springframework.expression.spel.SpelEvaluationException;
import org.springframework.expression.spel.SpelMessage;
import org.springframework.expression.spel.SpelNode;
import org.springframework.expression.spel.SpelParserConfiguration;
import org.springframework.expression.spel.ast.SpelNodeImpl;
import org.springframework.expression.spel.support.StandardEvaluationContext;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;

A SpelExpression represents a parsed (valid) expression that is ready to be evaluated in a specified context. An expression can be evaluated standalone or in a specified context. During expression evaluation the context may be asked to resolve references to types, beans, properties, and methods. 
Author:
Andy Clement, Juergen Hoeller
Since:
3.0/**
 * A {@code SpelExpression} represents a parsed (valid) expression that is ready to be
 * evaluated in a specified context. An expression can be evaluated standalone or in a
 * specified context. During expression evaluation the context may be asked to resolve
 * references to types, beans, properties, and methods.
 *
 * @author Andy Clement
 * @author Juergen Hoeller
 * @since 3.0
 */
public class SpelExpression implements Expression {

	// Number of times to interpret an expression before compiling it
	private static final int INTERPRETED_COUNT_THRESHOLD = 100;

	// Number of times to try compiling an expression before giving up
	private static final int FAILED_ATTEMPTS_THRESHOLD = 100;


	private final String expression;

	private final SpelNodeImpl ast;

	private final SpelParserConfiguration configuration;

	// The default context is used if no override is supplied by the user
	@Nullable
	private EvaluationContext evaluationContext;

	// Holds the compiled form of the expression (if it has been compiled)
	@Nullable
	private volatile CompiledExpression compiledAst;

	// Count of many times as the expression been interpreted - can trigger compilation
	// when certain limit reached
	private final AtomicInteger interpretedCount = new AtomicInteger();

	// The number of times compilation was attempted and failed - enables us to eventually
	// give up trying to compile it when it just doesn't seem to be possible.
	private final AtomicInteger failedAttempts = new AtomicInteger();


	
Construct an expression, only used by the parser.
/**
	 * Construct an expression, only used by the parser.
	 */
	public SpelExpression(String expression, SpelNodeImpl ast, SpelParserConfiguration configuration) {
		this.expression = expression;
		this.ast = ast;
		this.configuration = configuration;
	}


	
Set the evaluation context that will be used if none is specified on an evaluation call.
Params:
evaluationContext – the evaluation context to use/**
	 * Set the evaluation context that will be used if none is specified on an evaluation call.
	 * @param evaluationContext the evaluation context to use
	 */
	public void setEvaluationContext(EvaluationContext evaluationContext) {
		this.evaluationContext = evaluationContext;
	}

	
Return the default evaluation context that will be used if none is supplied on an evaluation call.
Returns:
the default evaluation context/**
	 * Return the default evaluation context that will be used if none is supplied on an evaluation call.
	 * @return the default evaluation context
	 */
	public EvaluationContext getEvaluationContext() {
		if (this.evaluationContext == null) {
			this.evaluationContext = new StandardEvaluationContext();
		}
		return this.evaluationContext;
	}


	// implementing Expression

	@Override
	public String getExpressionString() {
		return this.expression;
	}

	@Override
	@Nullable
	public Object getValue() throws EvaluationException {
		CompiledExpression compiledAst = this.compiledAst;
		if (compiledAst != null) {
			try {
				EvaluationContext context = getEvaluationContext();
				return compiledAst.getValue(context.getRootObject().getValue(), context);
			}
			catch (Throwable ex) {
				// If running in mixed mode, revert to interpreted
				if (this.configuration.getCompilerMode() == SpelCompilerMode.MIXED) {
					this.compiledAst = null;
					this.interpretedCount.set(0);
				}
				else {
					// Running in SpelCompilerMode.immediate mode - propagate exception to caller
					throw new SpelEvaluationException(ex, SpelMessage.EXCEPTION_RUNNING_COMPILED_EXPRESSION);
				}
			}
		}

		ExpressionState expressionState = new ExpressionState(getEvaluationContext(), this.configuration);
		Object result = this.ast.getValue(expressionState);
		checkCompile(expressionState);
		return result;
	}

	@SuppressWarnings("unchecked")
	@Override
	@Nullable
	public <T> T getValue(@Nullable Class<T> expectedResultType) throws EvaluationException {
		CompiledExpression compiledAst = this.compiledAst;
		if (compiledAst != null) {
			try {
				EvaluationContext context = getEvaluationContext();
				Object result = compiledAst.getValue(context.getRootObject().getValue(), context);
				if (expectedResultType == null) {
					return (T) result;
				}
				else {
					return ExpressionUtils.convertTypedValue(
							getEvaluationContext(), new TypedValue(result), expectedResultType);
				}
			}
			catch (Throwable ex) {
				// If running in mixed mode, revert to interpreted
				if (this.configuration.getCompilerMode() == SpelCompilerMode.MIXED) {
					this.compiledAst = null;
					this.interpretedCount.set(0);
				}
				else {
					// Running in SpelCompilerMode.immediate mode - propagate exception to caller
					throw new SpelEvaluationException(ex, SpelMessage.EXCEPTION_RUNNING_COMPILED_EXPRESSION);
				}
			}
		}

		ExpressionState expressionState = new ExpressionState(getEvaluationContext(), this.configuration);
		TypedValue typedResultValue = this.ast.getTypedValue(expressionState);
		checkCompile(expressionState);
		return ExpressionUtils.convertTypedValue(
				expressionState.getEvaluationContext(), typedResultValue, expectedResultType);
	}

	@Override
	@Nullable
	public Object getValue(@Nullable Object rootObject) throws EvaluationException {
		CompiledExpression compiledAst = this.compiledAst;
		if (compiledAst != null) {
			try {
				return compiledAst.getValue(rootObject, getEvaluationContext());
			}
			catch (Throwable ex) {
				// If running in mixed mode, revert to interpreted
				if (this.configuration.getCompilerMode() == SpelCompilerMode.MIXED) {
					this.compiledAst = null;
					this.interpretedCount.set(0);
				}
				else {
					// Running in SpelCompilerMode.immediate mode - propagate exception to caller
					throw new SpelEvaluationException(ex, SpelMessage.EXCEPTION_RUNNING_COMPILED_EXPRESSION);
				}
			}
		}

		ExpressionState expressionState =
				new ExpressionState(getEvaluationContext(), toTypedValue(rootObject), this.configuration);
		Object result = this.ast.getValue(expressionState);
		checkCompile(expressionState);
		return result;
	}

	@SuppressWarnings("unchecked")
	@Override
	@Nullable
	public <T> T getValue(@Nullable Object rootObject, @Nullable Class<T> expectedResultType) throws EvaluationException {
		CompiledExpression compiledAst = this.compiledAst;
		if (compiledAst != null) {
			try {
				Object result = compiledAst.getValue(rootObject, getEvaluationContext());
				if (expectedResultType == null) {
					return (T)result;
				}
				else {
					return ExpressionUtils.convertTypedValue(
							getEvaluationContext(), new TypedValue(result), expectedResultType);
				}
			}
			catch (Throwable ex) {
				// If running in mixed mode, revert to interpreted
				if (this.configuration.getCompilerMode() == SpelCompilerMode.MIXED) {
					this.compiledAst = null;
					this.interpretedCount.set(0);
				}
				else {
					// Running in SpelCompilerMode.immediate mode - propagate exception to caller
					throw new SpelEvaluationException(ex, SpelMessage.EXCEPTION_RUNNING_COMPILED_EXPRESSION);
				}
			}
		}

		ExpressionState expressionState =
				new ExpressionState(getEvaluationContext(), toTypedValue(rootObject), this.configuration);
		TypedValue typedResultValue = this.ast.getTypedValue(expressionState);
		checkCompile(expressionState);
		return ExpressionUtils.convertTypedValue(
				expressionState.getEvaluationContext(), typedResultValue, expectedResultType);
	}

	@Override
	@Nullable
	public Object getValue(EvaluationContext context) throws EvaluationException {
		Assert.notNull(context, "EvaluationContext is required");

		CompiledExpression compiledAst = this.compiledAst;
		if (compiledAst != null) {
			try {
				return compiledAst.getValue(context.getRootObject().getValue(), context);
			}
			catch (Throwable ex) {
				// If running in mixed mode, revert to interpreted
				if (this.configuration.getCompilerMode() == SpelCompilerMode.MIXED) {
					this.compiledAst = null;
					this.interpretedCount.set(0);
				}
				else {
					// Running in SpelCompilerMode.immediate mode - propagate exception to caller
					throw new SpelEvaluationException(ex, SpelMessage.EXCEPTION_RUNNING_COMPILED_EXPRESSION);
				}
			}
		}

		ExpressionState expressionState = new ExpressionState(context, this.configuration);
		Object result = this.ast.getValue(expressionState);
		checkCompile(expressionState);
		return result;
	}

	@SuppressWarnings("unchecked")
	@Override
	@Nullable
	public <T> T getValue(EvaluationContext context, @Nullable Class<T> expectedResultType) throws EvaluationException {
		Assert.notNull(context, "EvaluationContext is required");

		CompiledExpression compiledAst = this.compiledAst;
		if (compiledAst != null) {
			try {
				Object result = compiledAst.getValue(context.getRootObject().getValue(), context);
				if (expectedResultType != null) {
					return ExpressionUtils.convertTypedValue(context, new TypedValue(result), expectedResultType);
				}
				else {
					return (T) result;
				}
			}
			catch (Throwable ex) {
				// If running in mixed mode, revert to interpreted
				if (this.configuration.getCompilerMode() == SpelCompilerMode.MIXED) {
					this.compiledAst = null;
					this.interpretedCount.set(0);
				}
				else {
					// Running in SpelCompilerMode.immediate mode - propagate exception to caller
					throw new SpelEvaluationException(ex, SpelMessage.EXCEPTION_RUNNING_COMPILED_EXPRESSION);
				}
			}
		}

		ExpressionState expressionState = new ExpressionState(context, this.configuration);
		TypedValue typedResultValue = this.ast.getTypedValue(expressionState);
		checkCompile(expressionState);
		return ExpressionUtils.convertTypedValue(context, typedResultValue, expectedResultType);
	}

	@Override
	@Nullable
	public Object getValue(EvaluationContext context, @Nullable Object rootObject) throws EvaluationException {
		Assert.notNull(context, "EvaluationContext is required");

		CompiledExpression compiledAst = this.compiledAst;
		if (compiledAst != null) {
			try {
				return compiledAst.getValue(rootObject, context);
			}
			catch (Throwable ex) {
				// If running in mixed mode, revert to interpreted
				if (this.configuration.getCompilerMode() == SpelCompilerMode.MIXED) {
					this.compiledAst = null;
					this.interpretedCount.set(0);
				}
				else {
					// Running in SpelCompilerMode.immediate mode - propagate exception to caller
					throw new SpelEvaluationException(ex, SpelMessage.EXCEPTION_RUNNING_COMPILED_EXPRESSION);
				}
			}
		}

		ExpressionState expressionState = new ExpressionState(context, toTypedValue(rootObject), this.configuration);
		Object result = this.ast.getValue(expressionState);
		checkCompile(expressionState);
		return result;
	}

	@SuppressWarnings("unchecked")
	@Override
	@Nullable
	public <T> T getValue(EvaluationContext context, @Nullable Object rootObject, @Nullable Class<T> expectedResultType)
			throws EvaluationException {

		Assert.notNull(context, "EvaluationContext is required");

		CompiledExpression compiledAst = this.compiledAst;
		if (compiledAst != null) {
			try {
				Object result = compiledAst.getValue(rootObject, context);
				if (expectedResultType != null) {
					return ExpressionUtils.convertTypedValue(context, new TypedValue(result), expectedResultType);
				}
				else {
					return (T) result;
				}
			}
			catch (Throwable ex) {
				// If running in mixed mode, revert to interpreted
				if (this.configuration.getCompilerMode() == SpelCompilerMode.MIXED) {
					this.compiledAst = null;
					this.interpretedCount.set(0);
				}
				else {
					// Running in SpelCompilerMode.immediate mode - propagate exception to caller
					throw new SpelEvaluationException(ex, SpelMessage.EXCEPTION_RUNNING_COMPILED_EXPRESSION);
				}
			}
		}

		ExpressionState expressionState = new ExpressionState(context, toTypedValue(rootObject), this.configuration);
		TypedValue typedResultValue = this.ast.getTypedValue(expressionState);
		checkCompile(expressionState);
		return ExpressionUtils.convertTypedValue(context, typedResultValue, expectedResultType);
	}

	@Override
	@Nullable
	public Class<?> getValueType() throws EvaluationException {
		return getValueType(getEvaluationContext());
	}

	@Override
	@Nullable
	public Class<?> getValueType(@Nullable Object rootObject) throws EvaluationException {
		return getValueType(getEvaluationContext(), rootObject);
	}

	@Override
	@Nullable
	public Class<?> getValueType(EvaluationContext context) throws EvaluationException {
		Assert.notNull(context, "EvaluationContext is required");
		ExpressionState expressionState = new ExpressionState(context, this.configuration);
		TypeDescriptor typeDescriptor = this.ast.getValueInternal(expressionState).getTypeDescriptor();
		return (typeDescriptor != null ? typeDescriptor.getType() : null);
	}

	@Override
	@Nullable
	public Class<?> getValueType(EvaluationContext context, @Nullable Object rootObject) throws EvaluationException {
		ExpressionState expressionState = new ExpressionState(context, toTypedValue(rootObject), this.configuration);
		TypeDescriptor typeDescriptor = this.ast.getValueInternal(expressionState).getTypeDescriptor();
		return (typeDescriptor != null ? typeDescriptor.getType() : null);
	}

	@Override
	@Nullable
	public TypeDescriptor getValueTypeDescriptor() throws EvaluationException {
		return getValueTypeDescriptor(getEvaluationContext());
	}

	@Override
	@Nullable
	public TypeDescriptor getValueTypeDescriptor(@Nullable Object rootObject) throws EvaluationException {
		ExpressionState expressionState =
				new ExpressionState(getEvaluationContext(), toTypedValue(rootObject), this.configuration);
		return this.ast.getValueInternal(expressionState).getTypeDescriptor();
	}

	@Override
	@Nullable
	public TypeDescriptor getValueTypeDescriptor(EvaluationContext context) throws EvaluationException {
		Assert.notNull(context, "EvaluationContext is required");
		ExpressionState expressionState = new ExpressionState(context, this.configuration);
		return this.ast.getValueInternal(expressionState).getTypeDescriptor();
	}

	@Override
	@Nullable
	public TypeDescriptor getValueTypeDescriptor(EvaluationContext context, @Nullable Object rootObject)
			throws EvaluationException {

		Assert.notNull(context, "EvaluationContext is required");
		ExpressionState expressionState = new ExpressionState(context, toTypedValue(rootObject), this.configuration);
		return this.ast.getValueInternal(expressionState).getTypeDescriptor();
	}

	@Override
	public boolean isWritable(@Nullable Object rootObject) throws EvaluationException {
		return this.ast.isWritable(
				new ExpressionState(getEvaluationContext(), toTypedValue(rootObject), this.configuration));
	}

	@Override
	public boolean isWritable(EvaluationContext context) throws EvaluationException {
		Assert.notNull(context, "EvaluationContext is required");
		return this.ast.isWritable(new ExpressionState(context, this.configuration));
	}

	@Override
	public boolean isWritable(EvaluationContext context, @Nullable Object rootObject) throws EvaluationException {
		Assert.notNull(context, "EvaluationContext is required");
		return this.ast.isWritable(new ExpressionState(context, toTypedValue(rootObject), this.configuration));
	}

	@Override
	public void setValue(@Nullable Object rootObject, @Nullable Object value) throws EvaluationException {
		this.ast.setValue(
				new ExpressionState(getEvaluationContext(), toTypedValue(rootObject), this.configuration), value);
	}

	@Override
	public void setValue(EvaluationContext context, @Nullable Object value) throws EvaluationException {
		Assert.notNull(context, "EvaluationContext is required");
		this.ast.setValue(new ExpressionState(context, this.configuration), value);
	}

	@Override
	public void setValue(EvaluationContext context, @Nullable Object rootObject, @Nullable Object value)
			throws EvaluationException {

		Assert.notNull(context, "EvaluationContext is required");
		this.ast.setValue(new ExpressionState(context, toTypedValue(rootObject), this.configuration), value);
	}


	
Compile the expression if it has been evaluated more than the threshold number
of times to trigger compilation.
Params:
expressionState – the expression state used to determine compilation mode/**
	 * Compile the expression if it has been evaluated more than the threshold number
	 * of times to trigger compilation.
	 * @param expressionState the expression state used to determine compilation mode
	 */
	private void checkCompile(ExpressionState expressionState) {
		this.interpretedCount.incrementAndGet();
		SpelCompilerMode compilerMode = expressionState.getConfiguration().getCompilerMode();
		if (compilerMode != SpelCompilerMode.OFF) {
			if (compilerMode == SpelCompilerMode.IMMEDIATE) {
				if (this.interpretedCount.get() > 1) {
					compileExpression();
				}
			}
			else {
				// compilerMode = SpelCompilerMode.MIXED
				if (this.interpretedCount.get() > INTERPRETED_COUNT_THRESHOLD) {
					compileExpression();
				}
			}
		}
	}

	
Perform expression compilation. This will only succeed once exit descriptors for
all nodes have been determined. If the compilation fails and has failed more than
100 times the expression is no longer considered suitable for compilation.
Returns:
whether this expression has been successfully compiled/**
	 * Perform expression compilation. This will only succeed once exit descriptors for
	 * all nodes have been determined. If the compilation fails and has failed more than
	 * 100 times the expression is no longer considered suitable for compilation.
	 * @return whether this expression has been successfully compiled
	 */
	public boolean compileExpression() {
		CompiledExpression compiledAst = this.compiledAst;
		if (compiledAst != null) {
			// Previously compiled
			return true;
		}
		if (this.failedAttempts.get() > FAILED_ATTEMPTS_THRESHOLD) {
			// Don't try again
			return false;
		}

		synchronized (this) {
			if (this.compiledAst != null) {
				// Compiled by another thread before this thread got into the sync block
				return true;
			}
			SpelCompiler compiler = SpelCompiler.getCompiler(this.configuration.getCompilerClassLoader());
			compiledAst = compiler.compile(this.ast);
			if (compiledAst != null) {
				// Successfully compiled
				this.compiledAst = compiledAst;
				return true;
			}
			else {
				// Failed to compile
				this.failedAttempts.incrementAndGet();
				return false;
			}
		}
	}

	
Cause an expression to revert to being interpreted if it has been using a compiled
form. It also resets the compilation attempt failure count (an expression is normally no
longer considered compilable if it cannot be compiled after 100 attempts).
/**
	 * Cause an expression to revert to being interpreted if it has been using a compiled
	 * form. It also resets the compilation attempt failure count (an expression is normally no
	 * longer considered compilable if it cannot be compiled after 100 attempts).
	 */
	public void revertToInterpreted() {
		this.compiledAst = null;
		this.interpretedCount.set(0);
		this.failedAttempts.set(0);
	}

	
Return the Abstract Syntax Tree for the expression.
/**
	 * Return the Abstract Syntax Tree for the expression.
	 */
	public SpelNode getAST() {
		return this.ast;
	}

	
Produce a string representation of the Abstract Syntax Tree for the expression.
This should ideally look like the input expression, but properly formatted since any
unnecessary whitespace will have been discarded during the parse of the expression.
Returns:
the string representation of the AST/**
	 * Produce a string representation of the Abstract Syntax Tree for the expression.
	 * This should ideally look like the input expression, but properly formatted since any
	 * unnecessary whitespace will have been discarded during the parse of the expression.
	 * @return the string representation of the AST
	 */
	public String toStringAST() {
		return this.ast.toStringAST();
	}

	private TypedValue toTypedValue(@Nullable Object object) {
		return (object != null ? new TypedValue(object) : TypedValue.NULL);
	}

}