Advanced Plug-in Topics

This page discusses additional aspects of the Ice plug-in facility that may be of use to applications with special requirements.

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Plug-in Dependencies

If a plug-in has a dependency on another plug-in, you must ensure that Ice initializes the plug-ins in the proper order. Suppose that a custom plug-in depends on IceSSL; for example, it may need to make secure invocations on another server. We start with the following C++ configuration:

Ice.Plugin.IceSSL=IceSSL:createIceSSL
Ice.Plugin.MyPlugin=MyPlugin:createMyPlugin

The problem with this configuration is that it does not specify the order in which the plug-ins should be loaded and initialized. If the Ice run time happens to initialize MyPlugin first, the plug-in's initialize method will fail if it attempts to use the services of the uninitialized IceSSL plug-in.

To remedy the situation, we need to add one more property:

Ice.Plugin.IceSSL=IceSSL:createIceSSL
Ice.Plugin.MyPlugin=MyPlugin:createMyPlugin
Ice.PluginLoadOrder=IceSSL, MyPlugin

Using the Ice.PluginLoadOrder property we can guarantee that the plug-ins are loaded in the correct order.

Plug-ins added manually via the plug-in manager are appended to the end of the plug-in list, in order of addition. The last plug-in added is the first to be destroyed.

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The Plug-in Manager

PluginManager is the name of an internal Ice object that is responsible for managing all aspects of Ice plug-ins. This object supports a local Slice interface of the same name, and an application can obtain a reference to this object using the following communicator operation:

Slice
module Ice 
{
    local interface Communicator 
    {
        PluginManager getPluginManager();
        // ...
    }
}

The PluginManager interface offers three operations:

Slice
module Ice 
{
    local interface PluginManager 
    {
        void initializePlugins();
        Plugin getPlugin(string name);
        void addPlugin(string name, Plugin pi);
    }
}

The initializePlugins operation is used in special cases when an application needs to manually initialize one or more plug-ins, as discussed in the next section.

The getPlugin operation returns a reference to a specific plug-in. The name argument must match an installed plug-in, otherwise the operation raises NotRegisteredException. This operation is useful when a plug-in exports an interface that an application can use to query or customize its attributes or behavior.

Finally, addPlugin provides a way for an application to install a plug-in directly, without the use of a configuration property. This plug-in's initialize operation will be invoked if initializePlugins has not yet been called on the plug-in manager. If initializePlugins has already been called before a plug-in is added, Ice does not invoke initialize on the plug-in, but does invoke destroy during communicator destruction.

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Delayed Plug-in Initialization

It is sometimes necessary for an application to manually configure a plug-in prior to its initialization. For example, SSL keys are often protected by a passphrase, but a developer may be understandably reluctant to specify that passphrase in a configuration file because it would be exposed in clear text. The developer would likely prefer to configure the IceSSL plug-in with a password callback instead; however, this must be done before the plug-in is initialized and attempts to load the SSL key. The solution is to configure the Ice run time so that it postpones the initialization of its plug-ins:

Ice.InitPlugins=0

When Ice.InitPlugins is set to zero, initializing plug-ins becomes the application's responsibility. The example below demonstrates how to perform this initialization:

communicator = ...
auto pm = communicator->getPluginManager();
auto ssl = std::dynamic_pointer_cast<IceSSL::Plugin>(pm->getPlugin("IceSSL"));
ssl->setPasswordPrompt(...);
pm->initializePlugins();

communicator = ...
Ice::PluginManagerPtr pm = communicator->getPluginManager();
IceSSL::PluginPtr ssl = IceSSL::PluginPtr::dynamicCast(pm->getPlugin("IceSSL"));
ssl->setPasswordPrompt(...);
pm->initializePlugins();

After obtaining the IceSSL plug-in and establishing the password callback, the application invokes initializePlugins on the plug-in manager object to commence plug-in initialization.

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See Also