OperandStack.java
/*
* 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.bcel.verifier.structurals;
import java.util.ArrayList;
import org.apache.bcel.generic.ObjectType;
import org.apache.bcel.generic.ReferenceType;
import org.apache.bcel.generic.Type;
import org.apache.bcel.verifier.exc.AssertionViolatedException;
import org.apache.bcel.verifier.exc.StructuralCodeConstraintException;
/**
* This class implements a stack used for symbolic JVM stack simulation. [It's used as an operand stack substitute.]
* Elements of this stack are {@link Type} objects.
*/
public class OperandStack implements Cloneable {
/** We hold the stack information here. */
private ArrayList<Type> stack = new ArrayList<>();
/** The maximum number of stack slots this OperandStack instance may hold. */
private final int maxStack;
/**
* Creates an empty stack with a maximum of maxStack slots.
*/
public OperandStack(final int maxStack) {
this.maxStack = maxStack;
}
/**
* Creates an otherwise empty stack with a maximum of maxStack slots and the ObjectType 'obj' at the top.
*/
public OperandStack(final int maxStack, final ObjectType obj) {
this.maxStack = maxStack;
push(obj);
}
/**
* Clears the stack.
*/
public void clear() {
stack = new ArrayList<>();
}
/**
* Returns a deep copy of this object; that means, the clone operates on a new stack. However, the Type objects on the
* stack are shared.
*/
@Override
public Object clone() {
final OperandStack newstack = new OperandStack(this.maxStack);
@SuppressWarnings("unchecked") // OK because this.stack is the same type
final ArrayList<Type> clone = (ArrayList<Type>) this.stack.clone();
newstack.stack = clone;
return newstack;
}
/**
* Returns true if and only if this OperandStack equals another, meaning equal lengths and equal objects on the stacks.
*/
@Override
public boolean equals(final Object o) {
if (!(o instanceof OperandStack)) {
return false;
}
final OperandStack s = (OperandStack) o;
return this.stack.equals(s.stack);
}
/**
* Returns a (typed!) clone of this.
*
* @see #clone()
*/
public OperandStack getClone() {
return (OperandStack) clone();
}
/**
* @return a hash code value for the object.
*/
@Override
public int hashCode() {
return stack.hashCode();
}
/**
* Replaces all occurrences of u in this OperandStack instance with an "initialized" ObjectType.
*/
public void initializeObject(final UninitializedObjectType u) {
for (int i = 0; i < stack.size(); i++) {
if (stack.get(i) == u) {
stack.set(i, u.getInitialized());
}
}
}
/**
* Returns true IFF this OperandStack is empty.
*/
public boolean isEmpty() {
return stack.isEmpty();
}
/**
* Returns the number of stack slots this stack can hold.
*/
public int maxStack() {
return this.maxStack;
}
/**
* Merges another stack state into this instance's stack state. See the Java Virtual Machine Specification, Second
* Edition, page 146: 4.9.2 for details.
*/
public void merge(final OperandStack s) {
try {
if (slotsUsed() != s.slotsUsed() || size() != s.size()) {
throw new StructuralCodeConstraintException("Cannot merge stacks of different size:\nOperandStack A:\n" + this + "\nOperandStack B:\n" + s);
}
for (int i = 0; i < size(); i++) {
// If the object _was_ initialized and we're supposed to merge
// in some uninitialized object, we reject the code (see vmspec2, 4.9.4, last paragraph).
if (!(stack.get(i) instanceof UninitializedObjectType) && s.stack.get(i) instanceof UninitializedObjectType) {
throw new StructuralCodeConstraintException("Backwards branch with an uninitialized object on the stack detected.");
}
// Even harder, we're not initialized but are supposed to broaden
// the known object type
if (!stack.get(i).equals(s.stack.get(i)) && stack.get(i) instanceof UninitializedObjectType
&& !(s.stack.get(i) instanceof UninitializedObjectType)) {
throw new StructuralCodeConstraintException("Backwards branch with an uninitialized object on the stack detected.");
}
// on the other hand...
if (stack.get(i) instanceof UninitializedObjectType && !(s.stack.get(i) instanceof UninitializedObjectType)) { // that has been initialized by
// now
stack.set(i, ((UninitializedObjectType) stack.get(i)).getInitialized()); // note that.
}
if (!stack.get(i).equals(s.stack.get(i))) {
if (!(stack.get(i) instanceof ReferenceType) || !(s.stack.get(i) instanceof ReferenceType)) {
throw new StructuralCodeConstraintException("Cannot merge stacks of different types:\nStack A:\n" + this + "\nStack B:\n" + s);
}
stack.set(i, ((ReferenceType) stack.get(i)).getFirstCommonSuperclass((ReferenceType) s.stack.get(i)));
}
}
} catch (final ClassNotFoundException e) {
// FIXME: maybe not the best way to handle this
throw new AssertionViolatedException("Missing class: " + e, e);
}
}
/**
* Returns the element on top of the stack. The element is not popped off the stack!
*/
public Type peek() {
return peek(0);
}
/**
* Returns the element that's i elements below the top element; that means, iff i==0 the top element is returned. The
* element is not popped off the stack!
*/
public Type peek(final int i) {
return stack.get(size() - i - 1);
}
/**
* Returns the element on top of the stack. The element is popped off the stack.
*/
public Type pop() {
return stack.remove(size() - 1);
}
/**
* Pops i elements off the stack. Always returns null.
*
* @return Always returns null.
*/
public Type pop(final int count) {
for (int j = 0; j < count; j++) {
pop();
}
return null;
}
/**
* Pushes a Type object onto the stack.
*/
public void push(final Type type) {
if (type == null) {
throw new AssertionViolatedException("Cannot push NULL onto OperandStack.");
}
if (type == Type.BOOLEAN || type == Type.CHAR || type == Type.BYTE || type == Type.SHORT) {
throw new AssertionViolatedException("The OperandStack does not know about '" + type + "'; use Type.INT instead.");
}
if (slotsUsed() >= maxStack) {
throw new AssertionViolatedException("OperandStack too small, should have thrown proper Exception elsewhere. Stack: " + this);
}
stack.add(type);
}
/**
* Returns the size of this OperandStack; that means, how many Type objects there are.
*/
public int size() {
return stack.size();
}
/**
* Returns the number of stack slots used.
*
* @see #maxStack()
*/
public int slotsUsed() {
/*
* XXX change this to a better implementation using a variable that keeps track of the actual slotsUsed()-value
* monitoring all push()es and pop()s.
*/
int slots = 0;
for (int i = 0; i < stack.size(); i++) {
slots += peek(i).getSize();
}
return slots;
}
/**
* Returns a String representation of this OperandStack instance.
*/
@Override
public String toString() {
final StringBuilder sb = new StringBuilder();
sb.append("Slots used: ");
sb.append(slotsUsed());
sb.append(" MaxStack: ");
sb.append(maxStack);
sb.append(".\n");
for (int i = 0; i < size(); i++) {
sb.append(peek(i));
sb.append(" (Size: ");
sb.append(String.valueOf(peek(i).getSize()));
sb.append(")\n");
}
return sb.toString();
}
}