Lazy Start Bundles
The Equinox team has been working with the OSGi Alliance to standardize the Eclipse Lazy Start mechanism (i.e. Eclipse-LazyStart/Eclipse-AutoStart headers). The current plan is to add a lazy activation policy to the OSGi specification for the upcoming OSGi R4.1 specification. For more details on the current OSGi design proposal see OSGi design
Why should the Eclipse Community Care
The lazy activation policy built into the Equinox Framework is a powerful concept which allows Eclipse to launch with as few bundles active as possible. This concept is very important to Eclipse applications for both scalability and startup performance.
We already know that!! What has changed?
Yeah, this is old news. We have relied on this behavior for years.
This is true, but the support for this has only been available in the Equinox OSGi Framework. This means your bundles are tied to the Equinox Framework implementation and may not work as expected on other OSGi Framework implementations.
The following is a summary of what has changed in the OSGi proposal from what Equinox currently supports in version 3.2.
New Bundle-ActivationPolicy Header
Of course OSGi would not want to add a new header with Eclipse in the header name (e.g. Eclipse-LazyStart). A new header has been proposed Bundle-ActivationPolicy. See the OSGi design for more details.
Found some bugs
During the specification process a few bugs were found in the original lazy activation policy of Eclipse. For example, there is a well known ClassCircularityError bug bug 5874. This has lead to a subtle change in the way bundles are lazy activated. Consider the following example:
A system contains three Bundles: X, Y and Z. Bundle X has an interface x.X. Bundle Y has a class y.Y which implements x.X. Bundle Z has a class z.Z which extends y.Y. In this example, a request is made to load class z.Z. Class z.Z is thus the trigger class.
In Eclipse 3.2 the following occurs to lazy activate the bundles. When loading class z.Z the Framework notes that Bundle Z has a lazy activation policy. Before searching for the class the framework activates bundle Z then proceeds to find and define class z.Z. When defining z.Z the class y.Y is loaded from Bundle Y. The Framework notes that Bundle Y has a lazy activation policy. Before searching for the class the framework activates bundle Y then proceeds to find and define class y.Y. When defining y.Y the class x.X is loaded from Bundle X. The Framework notes that Bundle X has a lazy activation policy. Before searching for the class the framework activates bundle X then proceeds to find and define class x.X. Finally the class z.Z is returned to the client that caused it to load. In this case the activation order was be Z, Y, X.
This approach is flawed because bundles can be activated by a thread while that thread is in the process of defining a class. There are two important reasons this must be avoided.
- Deadlock issues - The thread defining a class must lock the classloader while it is defining a class (when it calls ClassLoader.defineClass). A thread locking a classloader of one bundle while activating another bundle will lead to deadlocks
- ClassCircularityErrors - Activating a bundle while defining a class can lead to scenarios that cause the thread defining the class to request the same class be loaded again while attempting to activate the bundle. This leads to the ClassCircularityErrors demonstrated in bug 5874.
The OSGi design for lazy activation uses the following approach to avoid these issues (using the same X, Y, Z bundles from above):
When loading class z.Z the Framework notes that Bundle Z has a lazy activation policy and adds it to the set of bundles that must be activated after the trigger class (z.Z) is defined. When defining z.Z the class y.Y is loaded from Bundle Y. The Framework notes that Bundle Y has a lazy activation policy and adds it to the set of bundles that must be activated after the trigger class (z.Z) is defined. When defining y.Y the class x.X is loaded from Bundle X. The Framework notes that Bundle X has a lazy activation policy and adds it to the set of bundles that must be activated after the trigger class (z.Z) is defined. Finally the class z.Z is successfully defined. Now each bundle on the set is activated in LIFO order. In this case the activation order would be X, Y, Z.
Notice that the order of activation has been changed from Z, Y, X to X, Y, Z.
What do I need to change in my bundle
Should I change to the new Bundle-ActivationPolicy Header?
The Equinox Framework version 3.3 will continue to support the Eclipse-LazyStart and the deprecated Eclipse-AutoStart headers. Bundles developers that want their bundles to work across other OSGi Framework implementations should add the Bundle-ActivationPolicy header to their manifest. In most cases you can simply add the following header
In many cases it is reasonable to also retain the old Eclipse-LazyStart or Eclipse-AutoStart headers if you need your bundle to continue to work on older versions of Eclipse.
What about the activation order changes?
Yeah in the above example the activation order was changed from Z, Y, X to X, Y, Z. That must break backwards compatibility?!!
Depending on the activation order in the above example to be Z, Y, X is very brittle and unpredictable. For example, nothing prevents another thread from accessing classes in Bundle X first causing it to be activated before Bundle Z is activated. The contract for lazy activation is that the bundle will be activated before a client uses a class loaded from that bundle. The new design still honors this contract but it improves upon the original Eclipse design to fix the deadlock and ClassCircularityError issues.
We expect very few (if any) bundles will need to worry about this change. Please let us know if you encounter a situation where this new approach breaks you.