The architecture of aglets

Find out about the inner workings of aglets, IBM Japan's Java-based autonomous software agent technology

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onClone() -- called on a clone after a clone operation

onArrival() -- called after a dispatch or a retraction

onActivation() -- called after an activation

Interaction between aglet and host

An aglet interacts with its environment (its aglet host) through an AgletContext object. An aglet can obtain a handle to its context by invoking getAgletContext(), a method it inherits from base class Aglet. The aglet context has methods such as createAglet() and retractAglet(), which allow an aglet to add new aglets (or get an old aglet back) to its local host.

Interaction between aglets

To interact with each other, aglets do not normally invoke each other's methods directly. Instead they go through AgletProxy objects, which serve as aglet representatives. For example, if a BossAglet wishes to make a request of an EmployeeAglet, the BossAglet obtains a handle to a proxy object that "represents" the EmployeeAglet. The BossAglet then makes a request by invoking a method in the EmployeeAglet's proxy, which in turn forwards the request to the actual EmployeeAglet.

The AgletProxy class contains methods that allow aglets to request other aglets to take actions, such as dispatch(), clone(), deactivate(), and dispose(). The aglet that has been requested to take an action can comply, refuse to comply, or decide to comply later.

The proxy also allows an aglet to send a message, either synchronously or asynchronously, to another aglet. A Message object is supplied for this purpose; it carries a String to indicate the kind of message plus one other optional piece of data, either a String or one of Java's primitive types. To send a message you create a Message object and pass it as a parameter to the sendMessage() or sendAsynchMessage() method of the proxy object.

An aglet must go through a proxy object to interact with an aglet, even if both aglets are in the same aglet host. The reason aglets aren't allowed to directly interact with one another is that the aglet's callback and initialization methods are public. These methods should be invoked only by the aglet host, but if an aglet could get a handle to another aglet, it could invoke that aglet's callback or initialization methods. An aglet could become very confused if another aglet inadvertently or maliciously invoked these methods directly.

The aglet being represented by a proxy might be local or remote, but the proxy object is always local. For example, if a BossAglet in Silicon Valley wants to communicate with an EmployeeAglet on a South Pacific island, the BossAglet gets a local AgletProxy object, which represents the remote EmployeeAglet. The BossAglet merely invokes methods in the local proxy, which in turn communicates across the network to the EmployeeAglet. Only aglets, not proxies, migrate across the network. A proxy communicates with a remote aglet that it represents by sending data across the network.

You get a proxy to an aglet in one of three ways, each of which involves invoking a method in the context object:

  1. By creating the aglet in the first place with createAglet(). (This returns a proxy object.)

  2. By searching through an enumeration of local proxies returned by getAgletProxies().

  3. By supplying an "aglet identifier" and, if remote, an aglet location as parameters to getAgletProxy(). (Every aglet, upon creation or cloning, is assigned a globally unique aglet identifier.)


Mobile-agent systems, such as aglets, require high levels of security, because they represent yet another way to transmit a malicious program. Before aglets can be used in practice, there must be an infrastructure of aglet hosts that prevent untrusted aglets from doing damage but provide trusted aglets with useful access to the host's resources. Security is amply provided for in Java's intrinsic architecture and in the extra security features of JDK 1.1, but as with applets, some attacks (such as denial of service by allocating memory until the host crashes) are still possible. Currently, the aglet hosts from IBM (named Tahiti and Fiji) place very severe security restrictions on the activities of any aglet that didn't originate locally.

Next month

Will aglets become as ubiquitous as their plastic cousins, which quietly perch on the ends of everyone's shoelaces? Aglets represent a good example of innovation on top of Java's network-oriented architecture, but what new benefits do they offer developers and end users that client/server, applets, and servlets don't already offer? In next month's Under The Hood, I will analyze the real-world utility of mobile agents in general and aglets in particular.

This month's article looks at aglets, an innovation developed by IBM Japan.

Bill Venners has been writing software professionally for 12 years. Based in Silicon Valley, he provides software consulting and training services under the name Artima Software Company. Over the years he has developed software for the consumer electronics, education, semiconductor, and life insurance industries. He has programmed in many languages on many platforms: assembly language on various microprocessors, C on Unix, C++ on Windows, Java on the Web. He is author of the book: Inside the Java Virtual Machine, published by McGraw-Hill.

Learn more about this topic

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  • The lean, mean virtual machine -- Gives an introduction to the Java virtual machine. Look here to see how the garbage collected heap fits in with the other parts of the Java virtual machine.
  • The Java class file lifestyle -- Gives an overview to the Java class file, the file format into which all Java programs are compiled.
  • Java's garbage-collected heap -- Gives an overview of garbage collection in general and the garbage-collected heap of the Java virtual machine in particular.
  • Bytecode basics -- Introduces the bytecodes of the Java virtual machine, and discusses primitive types, conversion operations, and stack operations in particular.
  • Floating Point Arithmetic -- Describes the Java virtual machine's floating-point support and the bytecodes that perform floating point operations.
  • Logic and Arithmetic -- Describes the Java virtual machine's support for logical and integer arithmetic, and the related bytecodes.
  • Objects and Arrays -- Describes how the Java virtual machine deals with objects and arrays, and discusses the relevant bytecodes.
  • Exceptions -- Describes how the Java virtual machine deals with exceptions, and discusses the relevant bytecodes.
  • Try-Finally -- Describes how the Java virtual machine implements try-finally clauses, and discusses the relevant bytecodes.
  • Control Flow -- Describes how the Java virtual machine implements control flow and discusses the relevant bytecodes.

This story, "The architecture of aglets" was originally published by JavaWorld.


Copyright © 1997 IDG Communications, Inc.

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