logback的UNDEFINED_PROPERTY属性源码执行流程解读

public static String substVars(String input, PropertyContainer pc0, PropertyContainer pc1) {
        try {
            return NodeToStringTransformer.substituteVariable(input, pc0, pc1);
        } catch (ScanException e) {
            throw new IllegalArgumentException("Failed to parse input [" + input + "]", e);
        }
    }

public static String substituteVariable(String input, PropertyContainer pc0, PropertyContainer pc1) throws ScanException {
        Node node = tokenizeAndParseString(input);
        NodeToStringTransformer nodeToStringTransformer = new NodeToStringTransformer(node, pc0, pc1);
        return nodeToStringTransformer.transform();
    }

public String transform() throws ScanException {
        StringBuilder stringBuilder = new StringBuilder();
        compileNode(node, stringBuilder, new Stack<Node>());
        return stringBuilder.toString();
    }

private void compileNode(Node inputNode, StringBuilder stringBuilder, Stack<Node> cycleCheckStack) throws ScanException {
        Node n = inputNode;
        while (n != null) {
            switch (n.type) {
            case LITERAL:
                handleLiteral(n, stringBuilder);
                break;
            case VARIABLE:
                handleVariable(n, stringBuilder, cycleCheckStack);
                break;
            }
            n = n.next;
        }
    }

private void handleVariable(Node n, StringBuilder stringBuilder, Stack<Node> cycleCheckStack) throws ScanException {
        // Check for recursion
        if (haveVisitedNodeAlready(n, cycleCheckStack)) {
            cycleCheckStack.push(n);
            String error = constructRecursionErrorMessage(cycleCheckStack);
            throw new IllegalArgumentException(error);
        }
        cycleCheckStack.push(n);
        StringBuilder keyBuffer = new StringBuilder();
        Node payload = (Node) n.payload;
        compileNode(payload, keyBuffer, cycleCheckStack);
        String key = keyBuffer.toString();
        String value = lookupKey(key);
        // empty values are considered valid
        if (value != null) {
            Node innerNode = tokenizeAndParseString(value);
            compileNode(innerNode, stringBuilder, cycleCheckStack);
            cycleCheckStack.pop();
            return;
        }
        // empty default literal is a valid value
        if (n.defaultPart == null) {
            stringBuilder.append(key + CoreConstants.UNDEFINED_PROPERTY_SUFFIX);
            cycleCheckStack.pop();
            return;
        }
        Node defaultPart = (Node) n.defaultPart;
        StringBuilder defaultPartBuffer = new StringBuilder();
        compileNode(defaultPart, defaultPartBuffer, cycleCheckStack);
        cycleCheckStack.pop();
        String defaultVal = defaultPartBuffer.toString();
        stringBuilder.append(defaultVal);
    }

public class CoreConstants {
    //......
    public static final String UNDEFINED_PROPERTY_SUFFIX = "_IS_UNDEFINED";
}