/*
 * 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.net.URL;
import java.net.URLClassLoader;
import java.util.Map;
import java.util.concurrent.atomic.AtomicInteger;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.asm.ClassWriter;
import org.springframework.asm.MethodVisitor;
import org.springframework.asm.Opcodes;
import org.springframework.expression.Expression;
import org.springframework.expression.spel.CodeFlow;
import org.springframework.expression.spel.CompiledExpression;
import org.springframework.expression.spel.SpelParserConfiguration;
import org.springframework.expression.spel.ast.SpelNodeImpl;
import org.springframework.lang.Nullable;
import org.springframework.util.ClassUtils;
import org.springframework.util.ConcurrentReferenceHashMap;
import org.springframework.util.ReflectionUtils;
import org.springframework.util.StringUtils;

A SpelCompiler will take a regular parsed expression and create (and load) a class
containing byte code that does the same thing as that expression. The compiled form of
an expression will evaluate far faster than the interpreted form.
The SpelCompiler is not currently handling all expression types but covers many of
the common cases. The framework is extensible to cover more cases in the future. For
absolute maximum speed there is *no checking* in the compiled code. The compiled
version of the expression uses information learned during interpreted runs of the
expression when it generates the byte code. For example if it knows that a particular
property dereference always seems to return a Map then it will generate byte code that
expects the result of the property dereference to be a Map. This ensures maximal
performance but should the dereference result in something other than a map, the
compiled expression will fail - like a ClassCastException would occur if passing data
of an unexpected type in a regular Java program.
Due to the lack of checking there are likely some expressions that should never be compiled, for example if an expression is continuously dealing with different types of data. Due to these cases the compiler is something that must be selectively turned on for an associated SpelExpressionParser (through the SpelParserConfiguration object), it is not on by default. 
Individual expressions can be compiled by calling SpelCompiler.compile(expression). 
Author:
Andy Clement, Juergen Hoeller
Since:
4.1/**
 * A SpelCompiler will take a regular parsed expression and create (and load) a class
 * containing byte code that does the same thing as that expression. The compiled form of
 * an expression will evaluate far faster than the interpreted form.
 *
 * <p>The SpelCompiler is not currently handling all expression types but covers many of
 * the common cases. The framework is extensible to cover more cases in the future. For
 * absolute maximum speed there is *no checking* in the compiled code. The compiled
 * version of the expression uses information learned during interpreted runs of the
 * expression when it generates the byte code. For example if it knows that a particular
 * property dereference always seems to return a Map then it will generate byte code that
 * expects the result of the property dereference to be a Map. This ensures maximal
 * performance but should the dereference result in something other than a map, the
 * compiled expression will fail - like a ClassCastException would occur if passing data
 * of an unexpected type in a regular Java program.
 *
 * <p>Due to the lack of checking there are likely some expressions that should never be
 * compiled, for example if an expression is continuously dealing with different types of
 * data. Due to these cases the compiler is something that must be selectively turned on
 * for an associated SpelExpressionParser (through the {@link SpelParserConfiguration}
 * object), it is not on by default.
 *
 * <p>Individual expressions can be compiled by calling {@code SpelCompiler.compile(expression)}.
 *
 * @author Andy Clement
 * @author Juergen Hoeller
 * @since 4.1
 */
public final class SpelCompiler implements Opcodes {

	private static final int CLASSES_DEFINED_LIMIT = 100;

	private static final Log logger = LogFactory.getLog(SpelCompiler.class);

	// A compiler is created for each classloader, it manages a child class loader of that
	// classloader and the child is used to load the compiled expressions.
	private static final Map<ClassLoader, SpelCompiler> compilers = new ConcurrentReferenceHashMap<>();


	// The child ClassLoader used to load the compiled expression classes
	private volatile ChildClassLoader childClassLoader;

	// Counter suffix for generated classes within this SpelCompiler instance
	private final AtomicInteger suffixId = new AtomicInteger(1);


	private SpelCompiler(@Nullable ClassLoader classloader) {
		this.childClassLoader = new ChildClassLoader(classloader);
	}


	
Attempt compilation of the supplied expression. A check is made to see
if it is compilable before compilation proceeds. The check involves
visiting all the nodes in the expression AST and ensuring enough state
is known about them that bytecode can be generated for them.
Params:
expression – the expression to compile
Returns:
an instance of the class implementing the compiled expression, or null if compilation is not possible/**
	 * Attempt compilation of the supplied expression. A check is made to see
	 * if it is compilable before compilation proceeds. The check involves
	 * visiting all the nodes in the expression AST and ensuring enough state
	 * is known about them that bytecode can be generated for them.
	 * @param expression the expression to compile
	 * @return an instance of the class implementing the compiled expression,
	 * or {@code null} if compilation is not possible
	 */
	@Nullable
	public CompiledExpression compile(SpelNodeImpl expression) {
		if (expression.isCompilable()) {
			if (logger.isDebugEnabled()) {
				logger.debug("SpEL: compiling " + expression.toStringAST());
			}
			Class<? extends CompiledExpression> clazz = createExpressionClass(expression);
			if (clazz != null) {
				try {
					return ReflectionUtils.accessibleConstructor(clazz).newInstance();
				}
				catch (Throwable ex) {
					throw new IllegalStateException("Failed to instantiate CompiledExpression", ex);
				}
			}
		}

		if (logger.isDebugEnabled()) {
			logger.debug("SpEL: unable to compile " + expression.toStringAST());
		}
		return null;
	}

	private int getNextSuffix() {
		return this.suffixId.incrementAndGet();
	}

	
Generate the class that encapsulates the compiled expression and define it.
The generated class will be a subtype of CompiledExpression.
Params:
expressionToCompile – the expression to be compiled
Returns:
the expression call, or null if the decision was to opt out of compilation during code generation/**
	 * Generate the class that encapsulates the compiled expression and define it.
	 * The generated class will be a subtype of CompiledExpression.
	 * @param expressionToCompile the expression to be compiled
	 * @return the expression call, or {@code null} if the decision was to opt out of
	 * compilation during code generation
	 */
	@Nullable
	private Class<? extends CompiledExpression> createExpressionClass(SpelNodeImpl expressionToCompile) {
		// Create class outline 'spel/ExNNN extends org.springframework.expression.spel.CompiledExpression'
		String className = "spel/Ex" + getNextSuffix();
		ClassWriter cw = new ExpressionClassWriter();
		cw.visit(V1_8, ACC_PUBLIC, className, null, "org/springframework/expression/spel/CompiledExpression", null);

		// Create default constructor
		MethodVisitor mv = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
		mv.visitCode();
		mv.visitVarInsn(ALOAD, 0);
		mv.visitMethodInsn(INVOKESPECIAL, "org/springframework/expression/spel/CompiledExpression",
				"<init>", "()V", false);
		mv.visitInsn(RETURN);
		mv.visitMaxs(1, 1);
		mv.visitEnd();

		// Create getValue() method
		mv = cw.visitMethod(ACC_PUBLIC, "getValue",
				"(Ljava/lang/Object;Lorg/springframework/expression/EvaluationContext;)Ljava/lang/Object;", null,
				new String[] {"org/springframework/expression/EvaluationException"});
		mv.visitCode();

		CodeFlow cf = new CodeFlow(className, cw);

		// Ask the expression AST to generate the body of the method
		try {
			expressionToCompile.generateCode(mv, cf);
		}
		catch (IllegalStateException ex) {
			if (logger.isDebugEnabled()) {
				logger.debug(expressionToCompile.getClass().getSimpleName() +
						".generateCode opted out of compilation: " + ex.getMessage());
			}
			return null;
		}

		CodeFlow.insertBoxIfNecessary(mv, cf.lastDescriptor());
		if ("V".equals(cf.lastDescriptor())) {
			mv.visitInsn(ACONST_NULL);
		}
		mv.visitInsn(ARETURN);

		mv.visitMaxs(0, 0);  // not supplied due to COMPUTE_MAXS
		mv.visitEnd();
		cw.visitEnd();

		cf.finish();

		byte[] data = cw.toByteArray();
		// TODO need to make this conditionally occur based on a debug flag
		// dump(expressionToCompile.toStringAST(), clazzName, data);
		return loadClass(StringUtils.replace(className, "/", "."), data);
	}

	
Load a compiled expression class. Makes sure the classloaders aren't used too much
because they anchor compiled classes in memory and prevent GC. If you have expressions
continually recompiling over time then by replacing the classloader periodically
at least some of the older variants can be garbage collected.
Params:
name – the name of the class
bytes – the bytecode for the class
Returns:
the Class object for the compiled expression/**
	 * Load a compiled expression class. Makes sure the classloaders aren't used too much
	 * because they anchor compiled classes in memory and prevent GC. If you have expressions
	 * continually recompiling over time then by replacing the classloader periodically
	 * at least some of the older variants can be garbage collected.
	 * @param name the name of the class
	 * @param bytes the bytecode for the class
	 * @return the Class object for the compiled expression
	 */
	@SuppressWarnings("unchecked")
	private Class<? extends CompiledExpression> loadClass(String name, byte[] bytes) {
		ChildClassLoader ccl = this.childClassLoader;
		if (ccl.getClassesDefinedCount() >= CLASSES_DEFINED_LIMIT) {
			synchronized (this) {
				ChildClassLoader currentCcl = this.childClassLoader;
				if (ccl == currentCcl) {
					// Still the same ClassLoader that needs to be replaced...
					ccl = new ChildClassLoader(ccl.getParent());
					this.childClassLoader = ccl;
				}
				else {
					// Already replaced by some other thread, let's pick it up.
					ccl = currentCcl;
				}
			}
		}
		return (Class<? extends CompiledExpression>) ccl.defineClass(name, bytes);
	}


	
Factory method for compiler instances. The returned SpelCompiler will
attach a class loader as the child of the given class loader and this
child will be used to load compiled expressions.
Params:
classLoader – the ClassLoader to use as the basis for compilation
Returns:
a corresponding SpelCompiler instance/**
	 * Factory method for compiler instances. The returned SpelCompiler will
	 * attach a class loader as the child of the given class loader and this
	 * child will be used to load compiled expressions.
	 * @param classLoader the ClassLoader to use as the basis for compilation
	 * @return a corresponding SpelCompiler instance
	 */
	public static SpelCompiler getCompiler(@Nullable ClassLoader classLoader) {
		ClassLoader clToUse = (classLoader != null ? classLoader : ClassUtils.getDefaultClassLoader());
		// Quick check for existing compiler without lock contention
		SpelCompiler compiler = compilers.get(clToUse);
		if (compiler == null) {
			// Full lock now since we're creating a child ClassLoader
			synchronized (compilers) {
				compiler = compilers.get(clToUse);
				if (compiler == null) {
					compiler = new SpelCompiler(clToUse);
					compilers.put(clToUse, compiler);
				}
			}
		}
		return compiler;
	}

	
Request that an attempt is made to compile the specified expression.
It may fail if components of the expression are not suitable for compilation
or the data types involved are not suitable for compilation. Used for testing.
Params:
expression – the expression to compile
Returns:
true if the expression was successfully compiled, false otherwise/**
	 * Request that an attempt is made to compile the specified expression.
	 * It may fail if components of the expression are not suitable for compilation
	 * or the data types involved are not suitable for compilation. Used for testing.
	 * @param expression the expression to compile
	 * @return {@code true} if the expression was successfully compiled,
	 * {@code false} otherwise
	 */
	public static boolean compile(Expression expression) {
		return (expression instanceof SpelExpression && ((SpelExpression) expression).compileExpression());
	}

	
Request to revert to the interpreter for expression evaluation.
Any compiled form is discarded but can be recreated by later recompiling again.
Params:
expression – the expression/**
	 * Request to revert to the interpreter for expression evaluation.
	 * Any compiled form is discarded but can be recreated by later recompiling again.
	 * @param expression the expression
	 */
	public static void revertToInterpreted(Expression expression) {
		if (expression instanceof SpelExpression) {
			((SpelExpression) expression).revertToInterpreted();
		}
	}


	
A ChildClassLoader will load the generated compiled expression classes.
/**
	 * A ChildClassLoader will load the generated compiled expression classes.
	 */
	private static class ChildClassLoader extends URLClassLoader {

		private static final URL[] NO_URLS = new URL[0];

		private final AtomicInteger classesDefinedCount = new AtomicInteger(0);

		public ChildClassLoader(@Nullable ClassLoader classLoader) {
			super(NO_URLS, classLoader);
		}

		public Class<?> defineClass(String name, byte[] bytes) {
			Class<?> clazz = super.defineClass(name, bytes, 0, bytes.length);
			this.classesDefinedCount.incrementAndGet();
			return clazz;
		}

		public int getClassesDefinedCount() {
			return this.classesDefinedCount.get();
		}
	}


	
An ASM ClassWriter extension bound to the SpelCompiler's ClassLoader.
/**
	 * An ASM ClassWriter extension bound to the SpelCompiler's ClassLoader.
	 */
	private class ExpressionClassWriter extends ClassWriter {

		public ExpressionClassWriter() {
			super(ClassWriter.COMPUTE_MAXS | ClassWriter.COMPUTE_FRAMES);
		}

		@Override
		protected ClassLoader getClassLoader() {
			return childClassLoader;
		}
	}

}