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The AspectJ weaver applies aspects to Java classes. It can be used as a Java agent in order to apply load-time weaving (LTW) during class-loading and also contains the AspectJ runtime classes.
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/* *******************************************************************
 * Copyright (c) 2002 Palo Alto Research Center, Incorporated (PARC).
 * All rights reserved.
 * This program and the accompanying materials are made available
 * under the terms of the Eclipse Public License v 2.0
 * which accompanies this distribution and is available at
 * https://www.eclipse.org/org/documents/epl-2.0/EPL-2.0.txt
 *
 * Contributors:
 *     PARC     initial implementation
 * ******************************************************************/

package org.aspectj.weaver.bcel;

import org.aspectj.apache.bcel.Constants;
import org.aspectj.apache.bcel.generic.InstructionFactory;
import org.aspectj.apache.bcel.generic.InstructionHandle;
import org.aspectj.apache.bcel.generic.InstructionList;
import org.aspectj.apache.bcel.generic.ObjectType;
import org.aspectj.apache.bcel.generic.ReferenceType;
import org.aspectj.apache.bcel.generic.Type;
import org.aspectj.weaver.BCException;
import org.aspectj.weaver.Member;
import org.aspectj.weaver.MemberImpl;
import org.aspectj.weaver.UnresolvedType;
import org.aspectj.weaver.ast.And;
import org.aspectj.weaver.ast.Call;
import org.aspectj.weaver.ast.CallExpr;
import org.aspectj.weaver.ast.Expr;
import org.aspectj.weaver.ast.FieldGet;
import org.aspectj.weaver.ast.FieldGetCall;
import org.aspectj.weaver.ast.HasAnnotation;
import org.aspectj.weaver.ast.IExprVisitor;
import org.aspectj.weaver.ast.ITestVisitor;
import org.aspectj.weaver.ast.Instanceof;
import org.aspectj.weaver.ast.Literal;
import org.aspectj.weaver.ast.Not;
import org.aspectj.weaver.ast.Or;
import org.aspectj.weaver.ast.Test;
import org.aspectj.weaver.ast.Var;
import org.aspectj.weaver.internal.tools.MatchingContextBasedTest;

// we generate right to left, btw.
public final class BcelRenderer implements ITestVisitor, IExprVisitor {

	private InstructionList instructions;
	private InstructionFactory fact;
	private BcelWorld world;

	InstructionHandle sk, fk, next = null;

	private BcelRenderer(InstructionFactory fact, BcelWorld world) {
		super();
		this.fact = fact;
		this.world = world;
		this.instructions = new InstructionList();
	}

	// ---- renderers

	public static InstructionList renderExpr(InstructionFactory fact, BcelWorld world, Expr e) {
		BcelRenderer renderer = new BcelRenderer(fact, world);
		e.accept(renderer);
		return renderer.instructions;
	}

	public static InstructionList renderExpr(InstructionFactory fact, BcelWorld world, Expr e, Type desiredType) {
		BcelRenderer renderer = new BcelRenderer(fact, world);
		e.accept(renderer);
		InstructionList il = renderer.instructions;
		il.append(Utility.createConversion(fact, BcelWorld.makeBcelType(e.getType()), desiredType));
		return il;
	}

	public static InstructionList renderExprs(InstructionFactory fact, BcelWorld world, Expr[] es) {
		BcelRenderer renderer = new BcelRenderer(fact, world);
		for (int i = es.length - 1; i >= 0; i--) {
			es[i].accept(renderer);
		}
		return renderer.instructions;
	}

	/*
	 * Get the instructions representing this test.
	 *
	 * @param e test to render
	 *
	 * @param sk instructionHandle to jump to if our rendered check succeeds (typically start of advice)
	 *
	 * @param fk instructionHandle to jump to if our rendered check fails (typically after end of advice)
	 *
	 * @param next instructionHandle that will follow this generated code. Passing in null will generate one unnecessary GOTO
	 * instruction.
	 *
	 * @returns the instruction list representing this expression
	 */
	public static InstructionList renderTest(InstructionFactory fact, BcelWorld world, Test e, InstructionHandle sk,
			InstructionHandle fk, InstructionHandle next) {
		BcelRenderer renderer = new BcelRenderer(fact, world);
		renderer.recur(e, sk, fk, next);
		return renderer.instructions;
	}

	// ---- recurrers

	private void recur(Test e, InstructionHandle sk, InstructionHandle fk, InstructionHandle next) {
		this.sk = sk;
		this.fk = fk;
		this.next = next;
		e.accept(this);
	}

	// ---- test visitors

	public void visit(And e) {
		InstructionHandle savedFk = fk;
		recur(e.getRight(), sk, fk, next);
		InstructionHandle ning = instructions.getStart();
		recur(e.getLeft(), ning, savedFk, ning);
	}

	public void visit(Or e) {
		InstructionHandle savedSk = sk;
		recur(e.getRight(), sk, fk, next);
		recur(e.getLeft(), savedSk, instructions.getStart(), instructions.getStart());
	}

	public void visit(Not e) {
		recur(e.getBody(), fk, sk, next);
	}

	public void visit(Instanceof i) {
		instructions.insert(createJumpBasedOnBooleanOnStack());
		instructions.insert(Utility.createInstanceof(fact, (ReferenceType) BcelWorld.makeBcelType(i.getType())));
		i.getVar().accept(this);
	}

	public void visit(HasAnnotation hasAnnotation) {
		// in Java:
		// foo.class.isAnnotationPresent(annotationClass);
		// in bytecode:

		// ifnull? skip to the end if it is as getClass() will fail (see pr 257833)

		// load var onto the stack (done for us later)
		// invokevirtual java/lang/Object.getClass:()Ljava/lang/Class
		// ldc_w annotationClass
		// invokevirtual java/lang/Class.isAnnotationPresent:(Ljava/lang/Class;)Z
		InstructionList il = new InstructionList();

		// If it is null jump past the advice call
		il.append(InstructionFactory.createBranchInstruction(Constants.IFNULL, fk));

		// Load up the var again
		il.append(((BcelVar) hasAnnotation.getVar()).createLoad(fact));

		Member getClass = MemberImpl.method(UnresolvedType.OBJECT, 0, UnresolvedType.JL_CLASS, "getClass", UnresolvedType.NONE);
		il.append(Utility.createInvoke(fact, world, getClass));
		// aload annotationClass
		il.append(fact.createConstant(new ObjectType(hasAnnotation.getAnnotationType().getName())));
		// int annClassIndex = fact.getConstantPool().addClass(hasAnnotation.getAnnotationType().getSignature());
		// il.append(new LDC_W(annClassIndex));
		Member isAnnotationPresent = MemberImpl.method(UnresolvedType.JL_CLASS, 0, UnresolvedType.BOOLEAN, "isAnnotationPresent",
				new UnresolvedType[] { UnresolvedType.JL_CLASS });
		il.append(Utility.createInvoke(fact, world, isAnnotationPresent));
		il.append(createJumpBasedOnBooleanOnStack());
		instructions.insert(il);
		hasAnnotation.getVar().accept(this);
	}

	/*
	 * (non-Javadoc)
	 *
	 * @see org.aspectj.weaver.ast.ITestVisitor#visit(org.aspectj.weaver.internal.tools.MatchingContextBasedTest)
	 */
	public void visit(MatchingContextBasedTest matchingContextTest) {
		throw new UnsupportedOperationException("matching context extension not supported in bytecode weaving");
	}

	private InstructionList createJumpBasedOnBooleanOnStack() {
		InstructionList il = new InstructionList();
		if (sk == fk) {
			// don't bother generating if it doesn't matter
			if (sk != next) {
				il.insert(InstructionFactory.createBranchInstruction(Constants.GOTO, sk));
			}
			return il;
		}

		if (fk == next) {
			il.insert(InstructionFactory.createBranchInstruction(Constants.IFNE, sk));
		} else if (sk == next) {
			il.insert(InstructionFactory.createBranchInstruction(Constants.IFEQ, fk));
		} else {
			il.insert(InstructionFactory.createBranchInstruction(Constants.GOTO, sk));
			il.insert(InstructionFactory.createBranchInstruction(Constants.IFEQ, fk));
		}
		return il;
	}

	public void visit(Literal literal) {
		if (literal == Literal.FALSE) {
			throw new BCException("visiting a false expression");
		}
	}

	public void visit(Call call) {
		Member method = call.getMethod();
		// assert method.isStatic()
		Expr[] args = call.getArgs();
		InstructionList callIl = new InstructionList();
		for (int i = 0, len = args.length; i < len; i++) {
			// XXX only correct for static method calls
			Type desiredType = BcelWorld.makeBcelType(method.getParameterTypes()[i]);
			Expr arg = args[i];
			// if arg is null it is because we couldn't bind it properly, for example see 162135
			if (arg == null) {
				InstructionList iList = new InstructionList();
				iList.append(InstructionFactory.createNull(desiredType));
				callIl.append(iList);
			} else {
				callIl.append(renderExpr(fact, world, arg, desiredType));
			}
		}
		// System.out.println("rendered args: " + callIl);
		callIl.append(Utility.createInvoke(fact, world, method));
		callIl.append(createJumpBasedOnBooleanOnStack());
		instructions.insert(callIl);
	}

	public void visit(FieldGetCall fieldGetCall) {
		Member field = fieldGetCall.getField();
		Member method = fieldGetCall.getMethod();
		InstructionList il = new InstructionList();
		il.append(Utility.createGet(fact, field));
		// assert !method.isStatic()
		Expr[] args = fieldGetCall.getArgs();
		// System.out.println("args: " + Arrays.asList(args));
		il.append(renderExprs(fact, world, args));
		// System.out.println("rendered args: " + callIl);
		il.append(Utility.createInvoke(fact, world, method));
		il.append(createJumpBasedOnBooleanOnStack());
		instructions.insert(il);
	}

	// ---- expr visitors

	public void visit(Var var) {
		BcelVar bvar = (BcelVar) var;
		bvar.insertLoad(instructions, fact);
	}

	public void visit(FieldGet fieldGet) {
		Member field = fieldGet.getField();
		// assert field.isStatic()
		instructions.insert(Utility.createGet(fact, field));
	}

	public void visit(CallExpr call) {
		Member method = call.getMethod();
		// assert method.isStatic()
		Expr[] args = call.getArgs();
		InstructionList callIl = renderExprs(fact, world, args);
		callIl.append(Utility.createInvoke(fact, world, method));
		instructions.insert(callIl);
	}

}