DefaultExpressionEngine.java
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* http://www.apache.org/licenses/LICENSE-2.0
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* Unless required by applicable law or agreed to in writing, software
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package org.apache.commons.configuration2.tree;
import java.util.Collection;
import java.util.LinkedList;
import java.util.List;
import org.apache.commons.lang3.StringUtils;
/**
* <p>
* A default implementation of the {@code ExpressionEngine} interface providing the "native" expression
* language for hierarchical configurations.
* </p>
* <p>
* This class implements a rather simple expression language for navigating through a hierarchy of configuration nodes.
* It supports the following operations:
* </p>
* <ul>
* <li>Navigating from a node to one of its children using the child node delimiter, which is by the default a dot
* (".").</li>
* <li>Navigating from a node to one of its attributes using the attribute node delimiter, which by default follows the
* XPATH like syntax {@code [@<attributeName>]}.</li>
* <li>If there are multiple child or attribute nodes with the same name, a specific node can be selected using a
* numerical index. By default indices are written in parenthesis.</li>
* </ul>
* <p>
* As an example consider the following XML document:
* </p>
*
* <pre>
* <database>
* <tables>
* <table type="system">
* <name>users</name>
* <fields>
* <field>
* <name>lid</name>
* <type>long</name>
* </field>
* <field>
* <name>usrName</name>
* <type>java.lang.String</type>
* </field>
* ...
* </fields>
* </table>
* <table>
* <name>documents</name>
* <fields>
* <field>
* <name>docid</name>
* <type>long</type>
* </field>
* ...
* </fields>
* </table>
* ...
* </tables>
* </database>
* </pre>
*
* <p>
* If this document is parsed and stored in a hierarchical configuration object, for instance the key
* {@code tables.table(0).name} can be used to find out the name of the first table. In opposite
* {@code tables.table.name} would return a collection with the names of all available tables. Similarly the key
* {@code tables.table(1).fields.field.name} returns a collection with the names of all fields of the second table. If
* another index is added after the {@code field} element, a single field can be accessed:
* {@code tables.table(1).fields.field(0).name}. The key {@code tables.table(0)[@type]} would select the type attribute
* of the first table.
* </p>
* <p>
* This example works with the default values for delimiters and index markers. It is also possible to set custom values
* for these properties so that you can adapt a {@code DefaultExpressionEngine} to your personal needs.
* </p>
* <p>
* The concrete symbols used by an instance are determined by a {@link DefaultExpressionEngineSymbols} object passed to
* the constructor. By providing a custom symbols object the syntax for querying properties in a hierarchical
* configuration can be altered.
* </p>
* <p>
* Instances of this class are thread-safe and can be shared between multiple hierarchical configuration objects.
* </p>
*
* @since 1.3
*/
public class DefaultExpressionEngine implements ExpressionEngine {
/**
* A default instance of this class that is used as expression engine for hierarchical configurations per default.
*/
public static final DefaultExpressionEngine INSTANCE = new DefaultExpressionEngine(DefaultExpressionEngineSymbols.DEFAULT_SYMBOLS);
/** The symbols used by this instance. */
private final DefaultExpressionEngineSymbols symbols;
/** The matcher for node names. */
private final NodeMatcher<String> nameMatcher;
/**
* Creates a new instance of {@code DefaultExpressionEngine} and initializes its symbols.
*
* @param syms the object with the symbols (must not be <b>null</b>)
* @throws IllegalArgumentException if the symbols are <b>null</b>
*/
public DefaultExpressionEngine(final DefaultExpressionEngineSymbols syms) {
this(syms, null);
}
/**
* Creates a new instance of {@code DefaultExpressionEngine} and initializes its symbols and the matcher for comparing
* node names. The passed in matcher is always used when the names of nodes have to be matched against parts of
* configuration keys.
*
* @param syms the object with the symbols (must not be <b>null</b>)
* @param nodeNameMatcher the matcher for node names; can be <b>null</b>, then a default matcher is used
* @throws IllegalArgumentException if the symbols are <b>null</b>
*/
public DefaultExpressionEngine(final DefaultExpressionEngineSymbols syms, final NodeMatcher<String> nodeNameMatcher) {
if (syms == null) {
throw new IllegalArgumentException("Symbols must not be null!");
}
symbols = syms;
nameMatcher = nodeNameMatcher != null ? nodeNameMatcher : NodeNameMatchers.EQUALS;
}
@Override
public String attributeKey(final String parentKey, final String attributeName) {
final DefaultConfigurationKey key = new DefaultConfigurationKey(this, parentKey);
key.appendAttribute(attributeName);
return key.toString();
}
/**
* {@inheritDoc} This implementation works similar to {@code nodeKey()}; however, each key returned by this method has
* an index (except for the root node). The parent key is prepended to the name of the current node in any case and
* without further checks. If it is <b>null</b>, only the name of the current node with its index is returned.
*/
@Override
public <T> String canonicalKey(final T node, final String parentKey, final NodeHandler<T> handler) {
final String nodeName = handler.nodeName(node);
final T parent = handler.getParent(node);
final DefaultConfigurationKey key = new DefaultConfigurationKey(this, parentKey);
key.append(StringUtils.defaultString(nodeName));
if (parent != null) {
// this is not the root key
key.appendIndex(determineIndex(node, parent, nodeName, handler));
}
return key.toString();
}
/**
* Determines the index of the given node based on its parent node.
*
* @param node the current node
* @param parent the parent node
* @param nodeName the name of the current node
* @param handler the node handler
* @param <T> the type of the nodes to be dealt with
* @return the index of this node
*/
private <T> int determineIndex(final T node, final T parent, final String nodeName, final NodeHandler<T> handler) {
return findChildNodesByName(handler, parent, nodeName).indexOf(node);
}
/**
* Returns a list with all child nodes of the given parent node which match the specified node name. The match is done
* using the current node name matcher.
*
* @param handler the {@code NodeHandler}
* @param parent the parent node
* @param nodeName the name of the current node
* @param <T> the type of the nodes to be dealt with
* @return a list with all matching child nodes
*/
private <T> List<T> findChildNodesByName(final NodeHandler<T> handler, final T parent, final String nodeName) {
return handler.getMatchingChildren(parent, nameMatcher, nodeName);
}
/**
* Finds the last existing node for an add operation. This method traverses the node tree along the specified key. The
* last existing node on this path is returned.
*
* @param <T> the type of the nodes to be dealt with
* @param keyIt the key iterator
* @param node the current node
* @param handler the node handler
* @return the last existing node on the given path
*/
protected <T> T findLastPathNode(final DefaultConfigurationKey.KeyIterator keyIt, final T node, final NodeHandler<T> handler) {
final String keyPart = keyIt.nextKey(false);
if (keyIt.hasNext()) {
if (!keyIt.isPropertyKey()) {
// Attribute keys can only appear as last elements of the path
throw new IllegalArgumentException("Invalid path for add operation: " + "Attribute key in the middle!");
}
final int idx = keyIt.hasIndex() ? keyIt.getIndex() : handler.getMatchingChildrenCount(node, nameMatcher, keyPart) - 1;
if (idx < 0 || idx >= handler.getMatchingChildrenCount(node, nameMatcher, keyPart)) {
return node;
}
return findLastPathNode(keyIt, findChildNodesByName(handler, node, keyPart).get(idx), handler);
}
return node;
}
/**
* Recursive helper method for evaluating a key. This method processes all facets of a configuration key, traverses the
* tree of properties and fetches the results of all matching properties.
*
* @param <T> the type of nodes to be dealt with
* @param keyPart the configuration key iterator
* @param node the current node
* @param results here the found results are stored
* @param handler the node handler
*/
protected <T> void findNodesForKey(final DefaultConfigurationKey.KeyIterator keyPart, final T node, final Collection<QueryResult<T>> results,
final NodeHandler<T> handler) {
if (!keyPart.hasNext()) {
results.add(QueryResult.createNodeResult(node));
} else {
final String key = keyPart.nextKey(false);
if (keyPart.isPropertyKey()) {
processSubNodes(keyPart, findChildNodesByName(handler, node, key), results, handler);
}
if (keyPart.isAttribute() && !keyPart.hasNext() && handler.getAttributeValue(node, key) != null) {
results.add(QueryResult.createAttributeResult(node, key));
}
}
}
/**
* Gets the {@code DefaultExpressionEngineSymbols} object associated with this instance.
*
* @return the {@code DefaultExpressionEngineSymbols} used by this engine
* @since 2.0
*/
public DefaultExpressionEngineSymbols getSymbols() {
return symbols;
}
/**
* {@inheritDoc} This implementation takes the given parent key, adds a property delimiter, and then adds the node's
* name. The name of the root node is a blank string. Note that no indices are returned.
*/
@Override
public <T> String nodeKey(final T node, final String parentKey, final NodeHandler<T> handler) {
if (parentKey == null) {
// this is the root node
return StringUtils.EMPTY;
}
final DefaultConfigurationKey key = new DefaultConfigurationKey(this, parentKey);
key.append(handler.nodeName(node), true);
return key.toString();
}
/**
* <p>
* Prepares Adding the property with the specified key.
* </p>
* <p>
* To be able to deal with the structure supported by hierarchical configuration implementations the passed in key is of
* importance, especially the indices it might contain. The following example should clarify this: Suppose the current
* node structure looks like the following:
* </p>
*
* <pre>
* tables
* +-- table
* +-- name = user
* +-- fields
* +-- field
* +-- name = uid
* +-- field
* +-- name = firstName
* ...
* +-- table
* +-- name = documents
* +-- fields
* ...
* </pre>
* <p>
* In this example a database structure is defined, e.g. all fields of the first table could be accessed using the key
* {@code tables.table(0).fields.field.name}. If now properties are to be added, it must be exactly specified at which
* position in the hierarchy the new property is to be inserted. So to add a new field name to a table it is not enough
* to say just
* </p>
*
* <pre>
* config.addProperty("tables.table.fields.field.name", "newField");
* </pre>
* <p>
* The statement given above contains some ambiguity. For instance it is not clear, to which table the new field should
* be added. If this method finds such an ambiguity, it is resolved by following the last valid path. Here this would be
* the last table. The same is true for the {@code field}; because there are multiple fields and no explicit index is
* provided, a new {@code name} property would be added to the last field - which is probably not what was desired.
* </p>
* <p>
* To make things clear explicit indices should be provided whenever possible. In the example above the exact table
* could be specified by providing an index for the {@code table} element as in {@code tables.table(1).fields}. By
* specifying an index it can also be expressed that at a given position in the configuration tree a new branch should
* be added. In the example above we did not want to add an additional {@code name} element to the last field of the
* table, but we want a complete new {@code field} element. This can be achieved by specifying an invalid index (like
* -1) after the element where a new branch should be created. Given this our example would run:
* </p>
*
* <pre>
* config.addProperty("tables.table(1).fields.field(-1).name", "newField");
* </pre>
* <p>
* With this notation it is possible to add new branches everywhere. We could for instance create a new {@code table}
* element by specifying
* </p>
*
* <pre>
* config.addProperty("tables.table(-1).fields.field.name", "newField2");
* </pre>
* <p>
* (Note that because after the {@code table} element a new branch is created indices in following elements are not
* relevant; the branch is new so there cannot be any ambiguities.)
* </p>
*
* @param <T> the type of the nodes to be dealt with
* @param root the root node of the nodes hierarchy
* @param key the key of the new property
* @param handler the node handler
* @return a data object with information needed for the add operation
*/
@Override
public <T> NodeAddData<T> prepareAdd(final T root, final String key, final NodeHandler<T> handler) {
final DefaultConfigurationKey.KeyIterator it = new DefaultConfigurationKey(this, key).iterator();
if (!it.hasNext()) {
throw new IllegalArgumentException("Key for add operation must be defined!");
}
final T parent = findLastPathNode(it, root, handler);
final List<String> pathNodes = new LinkedList<>();
while (it.hasNext()) {
if (!it.isPropertyKey()) {
throw new IllegalArgumentException("Invalid key for add operation: " + key + " (Attribute key in the middle.)");
}
pathNodes.add(it.currentKey());
it.next();
}
return new NodeAddData<>(parent, it.currentKey(), !it.isPropertyKey(), pathNodes);
}
/**
* Called by {@code findNodesForKey()} to process the sub nodes of the current node depending on the type of the current
* key part (children, attributes, or both).
*
* @param <T> the type of the nodes to be dealt with
* @param keyPart the key part
* @param subNodes a list with the sub nodes to process
* @param nodes the target collection
* @param handler the node handler
*/
private <T> void processSubNodes(final DefaultConfigurationKey.KeyIterator keyPart, final List<T> subNodes, final Collection<QueryResult<T>> nodes,
final NodeHandler<T> handler) {
if (keyPart.hasIndex()) {
if (keyPart.getIndex() >= 0 && keyPart.getIndex() < subNodes.size()) {
findNodesForKey((DefaultConfigurationKey.KeyIterator) keyPart.clone(), subNodes.get(keyPart.getIndex()), nodes, handler);
}
} else {
subNodes.forEach(node -> findNodesForKey((DefaultConfigurationKey.KeyIterator) keyPart.clone(), node, nodes, handler));
}
}
/**
* {@inheritDoc} This method supports the syntax as described in the class comment.
*/
@Override
public <T> List<QueryResult<T>> query(final T root, final String key, final NodeHandler<T> handler) {
final List<QueryResult<T>> results = new LinkedList<>();
findNodesForKey(new DefaultConfigurationKey(this, key).iterator(), root, results, handler);
return results;
}
}