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Oak Core

Oak API

The API for accessing core Oak functionality is located in the org.apache.jackrabbit.oak.api package and consists of the following key interfaces:

  • ContentRepository
  • ContentSession
  • Root / Tree

The ContentRepository interface represents an entire Oak content repository. The repository may local or remote, or a cluster of any size. These deployment details are all hidden behind this interface.

Starting and stopping ContentRepository instances is the responsibility of each particular deployment and not covered by these interfaces. Repository clients should use a deployment-specific mechanism (JNDI, OSGi service, etc.) to acquire references to ContentRepository instances.

All content in the repository is accessed through authenticated sessions acquired through the ContentRepository.login() method. The method takes explicit access credentials and other login details and, assuming the credentials are valid, returns a ContentSession instance that encapsulates this information. Session instances are Closeable and need to be closed to release associated resources once no longer used. The recommended access pattern is:

ContentRepository repository = ...;
ContentSession session = repository.login(...);
try {
    ...; // Use the session
} finally {
    session.close();
}

All ContentRepository and ContentSession instances are thread-safe.

The authenticated ContentSession gives you properly authorized access to the hierarchical content tree inside the repository through instances of the Root and Tree interfaces. The getCurrentRoot() method returns a snapshot of the current state of the content tree:

ContentSession session = ...;
Root root = session.getCurrentRoot();
Tree tree = root.getTree("/");

The returned Tree instance belongs to the client and its state is only modified in response to method calls made by the client. Tree instances are not thread-safe for write access, so writing clients need to ensure that they are not accessed concurrently from multiple threads. Tree instances are however thread-safe for read access, so implementations need to ensure that all reading clients see a coherent state.

Content trees are recursive data structures that consist of named properties and subtrees that share the same namespace, but are accessed through separate methods like outlined below:

Tree tree = ...;
for (PropertyState property : tree.getProperties()) {
    ...;
}
for (Tree subtree : tree.getChildren()) {
    ...;
}

The repository content snapshot exposed by a Tree instance may become invalid over time due to garbage collection of old content, at which point an outdated snapshot will start throwing IllegalStateExceptions to indicate that the snapshot is no longer available. To access more recent content, a client should either call ContentSession.getCurrentRoot() to acquire a fresh new content snapshot or use the refresh() method to update a given Root to the latest state of the content repository:

Root root = ...;
root.refresh();

In addition to reading repository content, the client can also make modifications to the content tree. Such content changes remain local to the particular Root instance (and related subtrees) until explicitly committed. For example, the following code creates and commits a new subtree containing nothing but a simple property:

ContentSession session = ...;
Root root = session.getCurrentRoot();
Tree tree = root.getTree("/");
Tree subtree = tree.addChild("hello");
subtree.setProperty("message", "Hello, World!");
root.commit();

Even other Root instances acquired from the same ContentSession won't see such changes until they've been committed and the other trees refreshed. This allows a client to track multiple parallel sets of changes with just a single authenticated session.

License

(see the top-level LICENSE.txt for full license details)

Collective work: Copyright 2012 The Apache Software Foundation.

Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to You under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

 http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.