下面我们开始从启动探寻 PMD 的源码设计。

pmd 的启动类为 PmdCli，作为命令行的启动器， 其依赖 picocli 作为控制台命令框架。
picocli 官网：https://picocli.info/

@Command(name = "checksum", mixinStandardHelpOptions = true, version = "checksum 4.0",
         description = "Prints the checksum (SHA-256 by default) of a file to STDOUT.")
class CheckSum implements Callable<Integer> {

    @Parameters(index = "0", description = "The file whose checksum to calculate.")
    private File file;

    @Option(names = {"-a", "--algorithm"}, description = "MD5, SHA-1, SHA-256, ...")
    private String algorithm = "SHA-256";

    @Override
    public Integer call() throws Exception { // your business logic goes here...
        byte[] fileContents = Files.readAllBytes(file.toPath());
        byte[] digest = MessageDigest.getInstance(algorithm).digest(fileContents);
        System.out.printf("%0" + (digest.length*2) + "x%n", new BigInteger(1, digest));
        return 0;
    }

    // this example implements Callable, so parsing, error handling and handling user
    // requests for usage help or version help can be done with one line of code.
    public static void main(String... args) {
        int exitCode = new CommandLine(new CheckSum()).execute(args);
        System.exit(exitCode);
    }
}

几个执行命令给抽出 Command 做解耦，实现方式还是很优雅的：

而且几个 Command 通过注解和继承，注册到 pico 的命令集中。
调用逻辑如下：

cli -> command -> PmdAnalysis -> pmd.runAndReturnStats() -> performAnalysis() -> performAnalysisImpl() -> launchAnalysis(analysisTask)) -> net.sourceforge.pmd.lang.impl.MultiThreadProcessor#processFiles -> net.sourceforge.pmd.lang.impl.PmdRunnable#processSource

private void processSource(FileAnalysisListener listener,
                               TextDocument textDocument,
                               RuleSets ruleSets) throws FileAnalysisException {

        SemanticErrorReporter reporter = SemanticErrorReporter.reportToLogger(task.getMessageReporter());
        @SuppressWarnings("PMD.CloseResource")
        LanguageProcessor processor = task.getLpRegistry().getProcessor(textDocument.getLanguageVersion().getLanguage());
        ParserTask parserTask = new ParserTask(textDocument,
                                               reporter,
                                               task.getLpRegistry());
        LanguageVersionHandler handler = processor.services();
		// 获取解析器
        Parser parser = handler.getParser();
		// 拿到抽象语法树根
        RootNode rootNode = parse(parser, parserTask);
        SemanticException semanticError = reporter.getFirstError();
        if (semanticError != null) {
            throw semanticError;
        }
		// 应用规则集
        ruleSets.apply(rootNode, listener);
    }

规则的调用核心： rule.apply(node, ctx);

    private void applyOnIndex(TreeIndex idx, Collection<? extends Rule> rules, FileAnalysisListener listener) {
        for (Rule rule : rules) {
            if (!ruleSetApplies(rule, currentLangVer)) {
                continue; // No point in even trying to apply the rule
            }
            
            RuleContext ctx = InternalApiBridge.createRuleContext(listener, rule);
            rule.start(ctx);
            try (TimedOperation rcto = TimeTracker.startOperation(TimedOperationCategory.RULE, rule.getName())) {

                int nodeCounter = 0;
                Iterator<? extends Node> targets = rule.getTargetSelector().getVisitedNodes(idx);
                while (targets.hasNext()) {
                    Node node = targets.next();

                    try {
                        nodeCounter++;
                        rule.apply(node, ctx);
                    } catch (RuntimeException e) {
                        reportOrRethrow(listener, rule, node, AssertionUtil.contexted(e), true);
                    } catch (StackOverflowError e) {
                        reportOrRethrow(listener, rule, node, AssertionUtil.contexted(e), SystemProps.isErrorRecoveryMode());
                    } catch (AssertionError e) {
                        reportOrRethrow(listener, rule, node, AssertionUtil.contexted(e), SystemProps.isErrorRecoveryMode());
                    }
                }
                
                rcto.close(nodeCounter);
            } finally {
                rule.end(ctx);
            }
        }
    }