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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) 1999-2001 Xerox Corporation,
 *               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:
 *     Xerox/PARC     initial implementation
 * ******************************************************************/


package org.aspectj.runtime.internal;

import java.util.Stack;

import org.aspectj.lang.NoAspectBoundException;
import org.aspectj.runtime.CFlow;
import org.aspectj.runtime.internal.cflowstack.ThreadStack;
import org.aspectj.runtime.internal.cflowstack.ThreadStackFactory;
import org.aspectj.runtime.internal.cflowstack.ThreadStackFactoryImpl;
import org.aspectj.runtime.internal.cflowstack.ThreadStackFactoryImpl11;

/*
 * How we benefit from ThreadLocal when it is available at runtime:
 *
 * When the CFlowStack class is loaded, we run its static initializer.  This checks the JVM
 * version number and loads an appropriate implementation of the ThreadStackFactory.
 * There are two possible implementations depending on whether this is a 1.1 or 1.2+ JVM.
 * Rather than doing a Class.forName for ThreadLocal and catching a ClassNotFoundEx in order
 * to determine the JVM version, we look at the java class version which I believe can help
 * us identify the Java level.
 *
 * In the 1.1 JVM case we use a factory implementation that does not use ThreadLocal storage.
 * In the 1.2+ JVM case we use a factory implementation that does use ThreadLocal storage.
 *
 * Once we have the factory set, whenever someone builds a CFlowStack object, we ask the
 * factory for a new stack proxy - this is an object that can return us the right stack
 * that we should use on a particular thread.  The reason we create the proxy in the ctor and
 * not lazily in the getThreadStack() method is because it means the getThreadStack() method in
 * this class does not have to be synchronized.
 *
 * When any of the methods in CFlowStack need to operate on the stack (peek/pop/etc), they
 * all delegate to getThreadStack() which asks the proxy for the right stack.  Depending on the
 * factory loaded to build the proxy, the call to proxy.getThreadStack() will return a threadlocal
 * based stack or it will call the original implementation of getThreadStack() which manages
 * a Hashtable of threads->stacks.
 *
 */

public class CFlowStack {

	private static ThreadStackFactory tsFactory;
	private ThreadStack stackProxy;

	static {
		selectFactoryForVMVersion();
	}

	public CFlowStack() {
		stackProxy = tsFactory.getNewThreadStack();
	}

    private Stack getThreadStack() {
    	return stackProxy.getThreadStack();
    }

	//XXX dangerous, try to remove
    public void push(Object obj) {
        getThreadStack().push(obj);
    }

    public void pushInstance(Object obj) {
        getThreadStack().push(new CFlow(obj));
    }

    public void push(Object[] obj) {
        getThreadStack().push(new CFlowPlusState(obj));
    }

    public void pop() {
        Stack s = getThreadStack();
        s.pop();
        if (s.isEmpty()) {
        	stackProxy.removeThreadStack();
        }
    }

    public Object peek() {
        Stack stack = getThreadStack();
        if (stack.isEmpty()) throw new org.aspectj.lang.NoAspectBoundException();
        return stack.peek();
    }

    public Object get(int index) {
        CFlow cf = peekCFlow();
        return (null == cf ? null : cf.get(index));
    }

    public Object peekInstance() {
    	CFlow cf = peekCFlow();
    	if (cf != null ) return cf.getAspect();
    	else throw new NoAspectBoundException();
    }

    public CFlow peekCFlow() {
        Stack stack = getThreadStack();
        if (stack.isEmpty()) return null;
        return (CFlow)stack.peek();
    }

    public CFlow peekTopCFlow() {
        Stack stack = getThreadStack();
        if (stack.isEmpty()) return null;
        return (CFlow)stack.elementAt(0);
    }

    public boolean isValid() {
        return !getThreadStack().isEmpty();
    }

	private static ThreadStackFactory getThreadLocalStackFactory()      { return new ThreadStackFactoryImpl(); }
	private static ThreadStackFactory getThreadLocalStackFactoryFor11() { return new ThreadStackFactoryImpl11(); }

	private static void selectFactoryForVMVersion() {
		String override = getSystemPropertyWithoutSecurityException("aspectj.runtime.cflowstack.usethreadlocal","unspecified");
		boolean useThreadLocalImplementation = false;
		if (override.equals("unspecified")) {
			String v = System.getProperty("java.class.version","0.0");
			// Java 1.2 is version 46.0 and above
			useThreadLocalImplementation = (v.compareTo("46.0") >= 0);
		} else {
			useThreadLocalImplementation = override.equals("yes") || override.equals("true");
		}
		// System.err.println("Trying to use thread local implementation? "+useThreadLocalImplementation);
		if (useThreadLocalImplementation) {
			tsFactory = getThreadLocalStackFactory();
		} else {
			tsFactory = getThreadLocalStackFactoryFor11();
		}
	}

	private static String getSystemPropertyWithoutSecurityException (String aPropertyName, String aDefaultValue) {
		try {
			return System.getProperty(aPropertyName, aDefaultValue);
		}
		catch (SecurityException ex) {
			return aDefaultValue;
		}
	}


	//  For debug ...
	public static String getThreadStackFactoryClassName() {
		return tsFactory.getClass().getName();
	}

}