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

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package org.apache.juneau.svl;

import static org.apache.juneau.internal.ThrowableUtils.*;

import java.io.*;

/**
 * Abstract superclass of all Simple Var Language variables.
 *
 * <p>
 * Vars are used to convert simple variables of the form <js>"$varName{varKey}"</js> into something else by the
 * {@link VarResolver} class.
 *
 * <p>
 * Subclasses must implement one of the following two methods:
 * <ul>
 * 	<li class='jm'>{@link #resolve(VarResolverSession,String)} - For simple vars.
 * 	<li class='jm'>{@link #resolveTo(VarResolverSession,Writer,String)} - For streamed vars.
 * </ul>
 *
 * <p>
 * Subclasses MUST implement a no-arg constructor so that class names can be passed to the
 * {@link VarResolverBuilder#vars(Class...)} method.
 * <br><b>They must also be thread safe!</b>
 *
 * <p>
 * Two direct abstract subclasses are provided to differentiated between simple and streamed vars:
 * <ul>
 * 	<li class='jac'>{@link SimpleVar}
 * 	<li class='jac'>{@link StreamedVar}
 * </ul>
 *
 * <h5 class='section'>See Also:</h5>
 * <ul>
 * 	<li class='link'>{@doc juneau-svl.SvlVariables}
 * </ul>
 */
public abstract class Var {

	private final String name;
	final boolean streamed;

	/**
	 * Constructor.
	 *
	 * @param name The name of this variable.
	 * @param streamed
	 * 	Whether this variable is 'streamed', meaning the {@link #resolveTo(VarResolverSession, Writer, String)} method
	 * 	is implemented.
	 * 	If <jk>false</jk>, then the {@link #resolve(VarResolverSession, String)} method is implemented.
	 */
	public Var(String name, boolean streamed) {
		this.name = name;
		this.streamed = streamed;

		if (name == null)
			illegalArg("Invalid var name.  Must consist of only uppercase and lowercase ASCII letters.");
		else for (int i = 0; i < name.length(); i++) {
		// Need to make sure only ASCII characters are used.
			char c = name.charAt(i);
			if (c < 'A' || c > 'z' || (c > 'Z' && c < 'a'))
				illegalArg("Invalid var name.  Must consist of only uppercase and lowercase ASCII letters.");
		}
	}

	/**
	 * Return the name of this variable.
	 *
	 * <p>
	 * For example, the system property variable returns <js>"S"</js> since the format of the variable is
	 * <js>"$S{system.property}"</js>.
	 *
	 * @return The name of this variable.
	 */
	protected String getName() {
		return name;
	}

	/**
	 * Returns whether nested variables are supported by this variable.
	 *
	 * <p>
	 * For example, in <js>"$X{$Y{xxx}}"</js>, $Y is a nested variable that will be resolved if this method returns
	 * <jk>true</jk>.
	 *
	 * <p>
	 * The default implementation of this method always returns <jk>true</jk>.
	 * Subclasses can override this method to override the default behavior.
	 *
	 * @return <jk>true</jk> if nested variables are supported by this variable.
	 */
	protected boolean allowNested() {
		return true;
	}

	/**
	 * Returns whether variables in the resolved contents of this variable should also be resolved.
	 *
	 * <p>
	 * For example, if <js>"$X{xxx}"</js> resolves to <js>"$Y{xxx}"</js>, then the $Y variable will be recursively
	 * resolved if this method returns <jk>true</jk>.
	 *
	 * <p>
	 * The default implementation of this method always returns <jk>true</jk>.
	 * <br>Subclasses can override this method to override the default behavior.
	 *
	 * <h5 class='topic'>Important Note</h5>
	 * <p>
	 * As a general rule, variables that resolve user-entered data should not be recursively resolved as this may
	 * cause a security hole.
	 *
	 * @return <jk>true</jk> if resolved variables should be recursively resolved.
	 */
	protected boolean allowRecurse() {
		return true;
	}

	/**
	 * The method called from {@link VarResolver}.
	 *
	 * <p>
	 * Can be overridden to intercept the request and do special handling.
	 * <br>Default implementation simply calls resolve(String).
	 *
	 * @param session The session object used for a single instance of a string resolution.
	 * @param arg The inside argument of the variable.
	 * @return The resolved value.
	 * @throws Exception Any exception can be thrown.
	 */
	protected String doResolve(VarResolverSession session, String arg) throws Exception {
		return resolve(session, arg);
	}

	/**
	 * The interface that needs to be implemented for subclasses of {@link SimpleVar}.
	 *
	 * @param session The session object used for a single instance of a var resolution.
	 * @param arg The inside argument of the variable.
	 * @return The resolved value.
	 * @throws Exception Any exception can be thrown.
	 */
	public abstract String resolve(VarResolverSession session, String arg) throws Exception;

	/**
	 * The interface that needs to be implemented for subclasses of {@link StreamedVar}.
	 *
	 * @param session The session object used for a single instance of a var resolution.
	 * @param w The writer to send the resolved value to.
	 * @param arg The inside argument of the variable.
	 * @throws Exception Any exception can be thrown.
	 */
	public abstract void resolveTo(VarResolverSession session, Writer w, String arg) throws Exception;
}