juneau-core/juneau-svl/src/main/java/org/apache/juneau/svl/VarResolver.java

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package org.apache.juneau.svl;

import java.io.*;
import java.util.*;

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

/**
 * Utility class for resolving variables of the form <js>"$X{key}"</js> in strings.
 *
 * <p>
 * Variables are of the form <code>$X{key}</code>, where <code>X</code> can consist of zero or more ASCII characters.
 * <br>The variable key can contain anything, even nested variables that get recursively resolved.
 *
 * <p>
 * Variables are defined through the {@link VarResolverBuilder#vars(Class[])} method.
 *
 * <p>
 * The {@link Var} interface defines how variables are converted to values.
 *
 * <h5 class='section'>Example:</h5>
 * <p class='bcode w800'>
 * 	<jk>public class</jk> SystemPropertiesVar <jk>extends</jk> SimpleVar {
 *
 * 		<jc>// Must have a no-arg constructor!</jc>
 * 		<jk>public</jk> SystemPropertiesVar() {
 * 			<jk>super</jk>(<js>"S"</js>);
 * 		}
 *
 * 		<ja>@Override</ja>
 * 		<jk>public</jk> String resolve(VarResolverSession session, String varVal) {
 * 			<jk>return</jk> System.<jsm>getProperty</jsm>(varVal);
 * 		}
 * 	}
 *
 * 	<jc>// Create a variable resolver that resolves system properties (e.g. "$S{java.home}")</jc>
 * 	VarResolver r = VarResolver.<jsm>create</jsm>().vars(SystemPropertiesVar.<jk>class</jk>).build();
 *
 * 	<jc>// Use it!</jc>
 * 	System.<jsf>out</jsf>.println(r.resolve(<js>"java.home is set to $S{java.home}"</js>));
 * </p>
 *
 * <h5 class='section'>See Also:</h5>
 * <ul>
 * 	<li class='link'>{@doc juneau-svl.VarResolvers}
 * </ul>
 */
public class VarResolver {

	/**
	 * Default string variable resolver with support for system properties and environment variables:
	 *
	 * <ul>
	 * 	<li><code>$S{key[,default]}</code> - {@link SystemPropertiesVar}
	 * 	<li><code>$E{key[,default]}</code> - {@link EnvVariablesVar}
	 * 	<li><code>$A{key[,default]}</code> - {@link ArgsVar}
	 * 	<li><code>$MF{key[,default]}</code> - {@link ManifestFileVar}
	 * 	<li><code>$IF{arg,then[,else]}</code> - {@link IfVar}
	 * 	<li><code>$SW{arg,pattern1:then1[,pattern2:then2...]}</code> - {@link SwitchVar}
	 * 	<li><code>$CO{arg[,arg2...]}</code> - {@link CoalesceVar}
	 * 	<li><code>$PM{arg,pattern}</code> - {@link PatternMatchVar}
	 * 	<li><code>$UC{arg}</code> - {@link UpperCaseVar}
	 * 	<li><code>$LC{arg}</code> - {@link LowerCaseVar}
	 * 	<li><code>$NE{arg}</code> - {@link NotEmptyVar}
	 * </ul>
	 *
	 * @see SystemPropertiesVar
	 * @see EnvVariablesVar
	 */
	public static final VarResolver DEFAULT = new VarResolverBuilder().defaultVars().build();

	final VarResolverContext ctx;

	/**
	 * Instantiates a new clean-slate {@link VarResolverBuilder} object.
	 *
	 * <p>
	 * This is equivalent to simply calling <code><jk>new</jk> VarResolverBuilder()</code>.
	 *
	 * @return A new {@link VarResolverBuilder} object.
	 */
	public static VarResolverBuilder create() {
		return new VarResolverBuilder();
	}

	/**
	 * Constructor.
	 *
	 * @param vars The var classes
	 * @param contextObjects
	 */
	VarResolver(Class<? extends Var>[] vars, Map<String,Object> contextObjects) {
		this.ctx = new VarResolverContext(vars, contextObjects);
	}

	/**
	 * Returns a new builder object using the settings in this resolver as a base.
	 *
	 * @return A new var resolver builder.
	 */
	public VarResolverBuilder builder() {
		return new VarResolverBuilder()
			.vars(ctx.getVars())
			.contextObjects(ctx.getContextObjects());
	}

	/**
	 * Returns the read-only properties on this variable resolver.
	 *
	 * @return The read-only properties on this variable resolver.
	 */
	public VarResolverContext getContext() {
		return ctx;
	}

	/**
	 * Creates a new resolver session with no session objects.
	 *
	 * <p>
	 * Session objects can be associated with the specified session using the {@link VarResolverSession#sessionObject(String, Object)}
	 * method.
	 *
	 * @return A new resolver session.
	 */
	public VarResolverSession createSession() {
		return new VarResolverSession(ctx, null);
	}

	/**
	 * Same as {@link #createSession()} except allows you to specify session objects as a map.
	 *
	 * @param sessionObjects The session objects to associate with the session.
	 * @return A new resolver session.
	 */
	public VarResolverSession createSession(Map<String,Object> sessionObjects) {
		return new VarResolverSession(ctx, sessionObjects);
	}

	/**
	 * Resolve variables in the specified string.
	 *
	 * <p>
	 * This is a shortcut for calling <code>createSession(<jk>null</jk>).resolve(s);</code>.
	 * <br>This method can only be used if the string doesn't contain variables that rely on the existence of session
	 * variables.
	 *
	 * @param s The input string.
	 * @return The string with variables resolved, or the same string if it doesn't contain any variables to resolve.
	 */
	public String resolve(String s) {
		return createSession(null).resolve(s);
	}

	/**
	 * Resolve variables in the specified string and sends the results to the specified writer.
	 *
	 * <p>
	 * This is a shortcut for calling <code>createSession(<jk>null</jk>).resolveTo(s, w);</code>.
	 * <br>This method can only be used if the string doesn't contain variables that rely on the existence of session
	 * variables.
	 *
	 * @param s The input string.
	 * @param w The writer to send the result to.
	 * @throws IOException
	 */
	public void resolveTo(String s, Writer w) throws IOException {
		createSession(null).resolveTo(s, w);
	}
}