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'>
 * 	<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 = <jk>new</jk> VarResolver().addVars(SystemPropertiesVar.<js>class</js>);
 *
 * 	<jc>// Use it!</jc>
 * 	System.<jsf>out</jsf>.println(r.resolve(<js>"java.home is set to $S{java.home}"</js>));
 * </p>
 *
 * <h6 class='topic'>Context objects</h6>
 *
 * Var resolvers can have zero or more context objects associated with them.
 *
 * <p>
 * Context objects are arbitrary objects associated with this var resolver, such as a <code>ConfigFile</code> object.
 * They can be any class type.
 *
 * <p>
 * Context objects can be retrieved by {@link Var} classes through the
 * {@link VarResolverSession#getSessionObject(Class, String)} method.
 *
 * <h6 class='topic'>Session objects</h6>
 *
 * Session objects are considered more ephemeral than context objects.
 * While a context object is unlikely to ever change, a session object may change on every use of the var resolver.
 * For example, the server API defines various <code>Var</code> objects that use the <code>RestRequest</code>
 * object as a session object for the duration of a single HTTP request.
 *
 * <p>
 * Session objects are used by calling the {@link #createSession()} or {@link #createSession(Map)} methods to create
 * an instance of a {@link VarResolverSession} object that contains {@link VarResolverSession#resolve(String)}
 * and {@link VarResolverSession#resolveTo(String,Writer)} methods that are identical to
 * {@link VarResolver#resolve(String)} and {@link VarResolver#resolveTo(String, Writer)} except that the
 * <code>Var</code> objects have access to the session objects through the
 * {@link VarResolverSession#getSessionObject(Class, String)} method.
 *
 * <p>
 * Session objects are specified through either the {@link #createSession(Map)} method or the
 * {@link VarResolverSession#sessionObject(String, Object)} methods.
 *
 * <h6 class='topic'>Cloning</h6>
 *
 * Var resolvers can be cloned by using the {@link #builder()} method.
 * Cloning a resolver will copy it's {@link Var} class names and context objects.
 *
 * <h5 class='section'>Example:</h5>
 * <p class='bcode'>
 * 	<jc>// Create a resolver that copies the default resolver and adds $C and $ARG vars.</jc>
 * 	VarResolver myVarResolver = VarResolver.<jsf>DEFAULT</jsf>.builder().vars(ConfigVar.<jk>class</jk>,
 * 		ArgsVar.<jk>class</jk>).build();
 * </p>
 *
 * @see org.apache.juneau.svl
 */
public class VarResolver {

	/**
	 * Default string variable resolver with support for system properties and environment variables:
	 *
	 * <ul>
	 * 	<li><code>$S{key}</code>,<code>$S{key,default}</code> - System properties.
	 * 	<li><code>$E{key}</code>,<code>$E{key,default}</code> - Environment variables.
	 * 	<li><code>$IF{booleanValue,thenValue[,elseValue]}</code> - If-else patterns.
	 * 	<li><code>$SW{test,matchPattern,thenValue[,matchPattern,thenValue][,elseValue]}</code> - Switch patterns.
	 * </ul>
	 *
	 * @see SystemPropertiesVar
	 * @see EnvVariablesVar
	 */
	public static final VarResolver DEFAULT = new VarResolverBuilder().defaultVars().build();

	final VarResolverContext ctx;

	/**
	 * Constructor.
	 *
	 * @param vars The var classes
	 * @param contextObjects
	 */
	public 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>.
	 * 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>.
	 * 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);
	}
}