juneau-core/juneau-config/src/main/java/org/apache/juneau/config/vars/ConfigVar.java

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package org.apache.juneau.config.vars;

import org.apache.juneau.config.*;
import org.apache.juneau.svl.*;

/**
 * Config file variable resolver.
 *
 * <p>
 * The format for this var is <js>"$C{key[,defaultValue]}"</js>.
 * See {@link Config#getString(String)} for the format of the key.
 *
 * <p>
 * This variable resolver requires that a {@link Config} object be set as a context object on the resolver or a
 * session object on the resolver session.
 *
 * <h5 class='section'>Example:</h5>
 * <p class='bcode w800'>
 * 	<jc>// Create a config object.</jc>
 * 	Config config = Config.<jsm>create</jsm>().name(<js>"MyConfig.cfg"</js>).build();
 *
 * 	<jc>// Create a variable resolver that resolves config file entries (e.g. "$C{MySection/myKey}")</jc>
 * 	VarResolver r = <jk>new</jk> VarResolver().addVars(ConfigVar.<js>class</js>)
 * 		.addContextObject(<jsf>SESSION_config</jsf>, configFile);
 *
 * 	<jc>// Use it!</jc>
 * 	System.<jsf>out</jsf>.println(r.resolve(<js>"Value for myKey in section MySection is $C{MySection/myKey}"</js>));
 * </p>
 *
 * <p>
 * Since this is a {@link SimpleVar}, any variables contained in the result will be recursively resolved.
 * Likewise, if the arguments contain any variables, those will be resolved before they are passed to this var.
 *
 * <ul class='seealso'>
 * 	<li class='link'>{@doc juneau-svl.VarResolvers}
 * 	<li class='link'>{@doc juneau-config.Variables}
 * </ul>
 */
public class ConfigVar extends DefaultingVar {

	/**
	 * The name of the session or context object that identifies the {@link Config} object.
	 */
	public static final String SESSION_config = "config";

	/** The name of this variable. */
	public static final String NAME = "C";

	/**
	 * Constructor.
	 */
	public ConfigVar() {
		super(NAME);
	}

	@Override /* Var */
	public String resolve(VarResolverSession session, String key) {
		return session.getSessionObject(Config.class, SESSION_config, true).getString(key);
	}
}