>Welcome to another edition of
Under The Hood
. Up to now, this column has focused on the inner workings of the Java virtual machine (JVM). I've written overviews of the JVM, the class file, and garbage collection, and have covered most of the JVM's bytecode instruction set. I have one final bytecode article coming in June, but this month I am going to begin expanding the column's scope. In the future, I plan to explore a broader array of topics. Each month I will focus on a particular aspect or application of Java technology, explain "how it works," and analyze what it means to Java developers.
This month's article looks at aglets, an innovation developed by IBM Japan.
Aglets: Not just for shoelaces anymore
According to Webster's Ninth New Collegiate Dictionary, an aglet is:
- aglet 1: the plain or ornamental tag covering the ends of a lace
- aglet 2: any of various ornamental studs, cords, or pins worn on clothing
In other words, aglets are those little plastic tubes on the ends of your shoelaces. Now, however, there is a new definition of the word aglet: a Java-based autonomous software agent.
As used here, a software agent is a program that can halt itself, ship itself to another computer on the network, and continue execution at the new computer. An agent doesn't restart execution from the beginning at the new computer; it continues where it left off. For example, imagine an agent that increments a counter starting with zero. If that agent counts from zero to ten, then halts and ships itself to another computer, it will not start counting again at zero. It will continue counting starting with ten, because that was where it left off when it halted at its previous computer.
Agents are autonomous because they decide where they will go and what they will do. They control their lifetimes. They can receive requests from external sources, such as other agents, but each individual agent decides whether or not to comply with external requests. Also, agents can decide to perform actions, such as travel across a network to a new computer, independent of any external request.
Aglets versus applets
The Java aglet extends the model of network-mobile code made famous by Java applets. Like an applet, the class files for an aglet can migrate across a network. But unlike applets, when an aglet migrates it also carries its state. An applet is code that can move across a network from a server to a client. An aglet is a running Java program (code and state) that can move from one host to another on a network. In addition, because an aglet carries its state wherever it goes, it can travel sequentially to many destinations on a network, including eventually returning back to its original host.
A Java aglet is similar to an applet in that it runs as a thread (or multiple threads) inside the context of a host Java application. To run applets, a Web browser fires off a Java application to host any applets it may encounter as the user browses from page to page. That application installs a security manager to enforce restrictions on the activities of any untrusted applets. To download an applet's class files, the application creates class loaders that know how to request class files from an HTTP server.
Likewise, an aglet requires a host Java application, an "aglet host," to be running on a computer before it can visit that computer. When aglets travel across a network, they migrate from one aglet host to another. Each aglet host installs a security manager to enforce restrictions on the activities of untrusted aglets. Hosts upload aglets through class loaders that know how to retrieve the class files and state of an aglet from a remote aglet host.
The aglet lifestyle
An aglet can experience many events in its life. It can be:
Created: a brand new aglet is born -- its state is initialized, its main thread starts executing
Cloned: a twin aglet is born -- the current state of the original is duplicated in the clone
Dispatched: an aglet travels to a new host -- the state goes with it
Retracted: an aglet, previously dispatched, is brought back from a remote host -- its state comes back with it
Deactivated: an aglet is put to sleep -- its state is stored on a disk somewhere
Activated: a deactivated aglet is brought back to life -- its state is restored from disk
Disposed of: an aglet dies -- its state is lost forever
Note that every activity besides creation and disposal involve either duplication, transmission across a network, or persistent storage of the aglet's state. Each of these activities uses the same process to get the state out of an aglet: serialization.
Serializing the state...
Aglet hosts use object serialization, available in JDK 1.1 or with the RMI (remote method invocation) add-on to JDK 1.0.2, to export the state of an aglet object to a stream of bytes. Through this process, the aglet object and the tree of serializable objects reachable from it, are written to a stream. An object is serializable if it implements either the
Serializable or the
Externalizable interface. In a reverse process, the state of the aglet can be reconstructed from the stream of bytes. Serialization allows an image of the heap (the heap's state) to be exported to a byte stream (such as a file) and then reconstructed from that byte stream.
...but not all of the state
The state of the execution stacks and program counters of the threads owned by the aglet are not serialized. Object serialization touches only data on the heap, not the stacks or the program counters. Thus when an aglet is dispatched, cloned, or deactivated, any relevant state sitting on any stack of a running aglet, as well as the current program counter for any thread, is lost.
In theory, a software agent should be able to migrate with all its state: heap, execution stack, and registers. Some will likely consider the inability of aglets to do this as a flaw in the aglet's implementation of mobile-agent theory. This feature of aglets arises out of the architecture of the JVM, which doesn't allow a program to directly access and manipulate execution stacks. This is part of the JVM's built-in security model. Unless there is a change to the JVM, aglets and any other mobile Java-based agent will be unable to carry the state of their execution stacks with them as they migrate.
Before it is serialized, an aglet must place on the heap everything it will need to know to be resurrected properly as a newly activated aglet, a freshly dispatched aglet, or a clone. It can't leave any of this information on the stack, because the stacks won't be reproduced in the aglet's new life. As a result, the aglet host informs an aglet that it is about to be serialized so that the aglet can prepare itself. When the aglet is informed of an impending serialization, it must place onto the heap any information it will need to continue its execution properly when it is resurrected.
From a practical standpoint, the inability of an aglet to migrate with its execution stacks is not an unreasonable limitation. It simply forces you to think a certain way when you write aglets. You can look at an aglet as a finite state machine with the heap as the sole repository of the machine's state. If at any point in an aglet's life you can know what state it is in by looking at its heap, then it can be serialized at any time. If not, then you must have a way to record sufficient information on the heap just prior to serialization such that you can continue properly when the aglet is resurrected.
Also, even though the inability to serialize execution stacks necessitates giving aglets a warning prior to serialization, such warnings probably are a good idea anyway. It is difficult to think of a case in which an aglet wouldn't want to know it was about to be serialized and why. It may need to finish some incomplete process before allowing the serialization, or it may want to refuse the action that requires the serialization. For example, if an agent is told it is about to be serialized and dispatched to an aglet host in Silicon Valley, it may refuse and decide instead to dispatch itself to a host on an island in the South Pacific.
How to write an aglet
The process of writing an aglet is in many ways similar to the process of writing an applet. To create an applet, you subclass class
Applet. To initialize an applet, you override the
init() method, the starting point for any applet. You can use
init() to build the user interface of the applet. If you wish, you can fire off other threads from
init(). If you do this, you also may override
start() to stop and restart your threads when the browser leaves and returns to the Web page. If you don't create any threads in
init(), your applet likely will get at least one thread just because class
Applet descends from class
Panel. The AWT user-interface library of which
Panel is a part will provide whatever threads are needed to run the user interface you create in
The aglet development and run-time environments provide a library of Java classes that support the creation and running of aglets. To create an aglet, you must subclass class
Aglet, which includes several methods you can override to customize the behavior of your aglet. The aglet's counterpart to the
init() method of applets is the
onCreation() method. To initialize an aglet, you override
onCreation() method is invoked only once in an aglet's lifetime and should be used only for initialization.
The aglet also has a
run() method, which represents the entry point for the aglet's main thread. This is similar to the
main() method of a Java application, except that
run() is invoked each time an aglet arrives at a new aglet host. For example, if you designed a
CatAglet that visits nine different aglet hosts looking for
onCreation() would be invoked only once, when the
CatAglet was first instantiated at its first host. Once
run() would be invoked. Each time the
CatAglet arrived at a new host, a method called
onArrival() would be invoked to perform any initialization. Once
run() would be invoked to get the aglet started again at the new host.
run() again each time an aglet is brought to life illustrates the inability of aglets to transmit the state of their execution stacks. For example, imagine a
run() method periodically invokes a method named
walk(). If, as it is walking, the
HealthyAglet is serialized and transmitted to another host, it wouldn't by default continue executing where it left off in
walk(). It would start over again at the beginning of
run(). Thus, when the aglet is informed that it is about to be serialized, it would need to record on the heap that it is walking -- perhaps in an instance variable of
HealthyAglet. That instance variable would be serialized and would migrate with the aglet. When
run() is invoked to start the aglet's new life, the
run() method would check the instance variable, see it was walking beforehand, and call
The callback model
Before any major event in an aglet's life, a "callback" method is invoked to allow the aglet to prepare for (or refuse to partake in) the event. This is how an aglet learns that it is about to be serialized. For example, before an aglet is dispatched to a new location, the aglet's
onDispatch() is invoked. This method indicates to an aglet that it is about to be sent to a new host, the URL of which is specified as a parameter to
onDispatch(). In the body of
onDispatch(), the aglet must decide whether or not to go. If the aglet decides it doesn't want to go, it throws an exception. If it decides to go, it must complete any unfinished business and prepare its state for serialization. When it returns from
onDispatch(), its state will be serialized and all its threads terminated. The class files and serialized state will then be sent to the new host, where the aglet will be resurrected.
onDispatch() is a "callback" method because the aglet host invokes it some time after another method,
dispatch(), is invoked. An aglet can invoke
dispatch() on itself or on another aglet. This callback model for aglets is similar to that of windowing user interfaces. To repaint an AWT component, for example, you invoke the component's
repaint() method. At some point later, the system calls back the component's
update() method, which in turn calls
Aglet class defines these five callback methods, which you can override to customize the behavior of your aglet:
onCloning() -- called before a clone operation
onDispatch() -- called before a dispatch
onReverting() -- called before a retraction
onDeactivating() -- called before a deactivation
onDisposing() -- called before a dispose operation (Unlike real life, an aglet can throw an exception if it doesn't want to die.)
For each of these processes, the
Aglet class has a corresponding method that triggers the action:
dispose(). Some time after these are called, the aglet host will invoke the appropriate callback method.
Each time an aglet begins execution at a host, the host invokes an initialization method on the aglet. When the initialization method returns, the host invokes
run(). Depending on the event that precipitated the aglet's new life, the aglet host will choose to invoke one of these four initialization methods:
onCreation() -- called the first time an aglet springs to life