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需求

最近想将自己的一个Java应用包装成命令行工具，看了几个库，最后选取了JCommander，结合AutoService库，实现了带子命令的工具，方便扩展新的子命令。

子命令放在同一包下，实现相同的接口，通过java.util.ServiceLocator加载。

Java命令行工具库

常用的几个库为:

• JCommander
  项目地址： https://github.com/cbeust/jcommander
  Star: 1010 Fork: 227
  文档地址： http://jcommander.org/

• picocli
  地址： https://github.com/remkop/picocli
  Star： 336， Fork： 32
  示例：https://github.com/kakawait/picocli-spring-boot-starter

• Commons CLI
  地址： https://commons.apache.org/proper/commons-cli/
  地址： 来自apache common的开源项目
  更新： 最后一次更新是1.5-SNAPSHOT，是在2017年6月8日

• Args4j
  项目地址： https://0github.com/kohsuke/args4j
  Star： 570 Fork： 151
  文档地址： http://args4j.kohsuke.org/sample.html
  活跃程度： 最后一次更新为2年之前

依赖库

        <!-- https://mvnrepository.com/artifact/com.google.auto.service/auto-service --><dependency><groupId>com.google.auto.service</groupId><artifactId>auto-service</artifactId><version>1.1.1</version></dependency><!-- https://mvnrepository.com/artifact/com.beust/jcommander --><dependency><groupId>com.beust</groupId><artifactId>jcommander</artifactId><version>1.82</version></dependency>

定义各个子命令

Create子命令:

@AutoService(Command.class)
@Parameters(commandNames = { CREATE_CMD },commandDescription = "create a new ebook."
)
@Getter
public class CreateCommand implements Command{@Parameter(names = { "--indexUrl", "-u" })public String indexUrl;@Overridepublic void execute() throws CommandException {System.out.println("create a new book from " + indexUrl);}
}

Fetch子命令：

@AutoService(Command.class)
@Parameters(commandNames = {FETCH_CMD},commandDescription = "fetch some articles from website."
)
@Getter
public class FetchCommand implements Command {@Parameter(names = {"--indexUrl", "-u"})public String indexUrl;@Overridepublic void execute() throws CommandException {System.out.println("fetch articles from " + indexUrl);}
}

都实现了接口Command:

public interface Command {default Collection<Command> commands() {return null;}void execute() throws CommandException;
}

注意，子命令支持嵌套的子命令。当然一般用不到。

主类CLI

public class CLI {static final String CREATE_CMD = "create";static final String FETCH_CMD = "fetch";@Parameter(names = { "-h", "--help" }, help = true)private boolean help;public void exec(String[] args) {final JCommander.Builder builder = JCommander.newBuilder().addObject(new CLI());       final JCommander jCommander = builder.build();ServiceLoader.load(Command.class).forEach(command -> CLI.registerCommand(jCommander, command));JCommander leafCommander = jCommander;try {jCommander.parse(args);final String rootVerb = jCommander.getParsedCommand();final JCommander rootCommander = jCommander.getCommands().get(rootVerb);if (rootCommander == null) {jCommander.usage();System.exit(1);}leafCommander = rootCommander;do {final String subVerb = leafCommander.getParsedCommand();final JCommander subCommander = leafCommander.getCommands().get(subVerb);if (subCommander != null)leafCommander = subCommander;elsebreak;} while (true);final Command command = (Command) leafCommander.getObjects().get(0);command.execute();} catch (final CommandException e) {System.err.printf("%1$s: %2$s. See '%1$s --help'.%n", leafCommander.getProgramName(), e.getMessage());System.exit(e.getStatus());} catch (final Exception e) {System.err.printf("%1$s: %2$s. See '%1$s --help'.%n", leafCommander.getProgramName(), e.getMessage());System.exit(1);}}private static final void registerCommand(final JCommander jCommander, final Command command) {jCommander.addCommand(command);final Parameters commandParameters = command.getClass().getAnnotation(Parameters.class);if (commandParameters == null || commandParameters.commandNames().length == 0)return;final JCommander subCommander = jCommander.getCommands().get(commandParameters.commandNames()[0]);final Collection<Command> subCommands = command.commands();if (subCommands != null)subCommands.forEach(subCommand -> CLI.registerCommand(subCommander, subCommand));}public static void main(final String[] args) {CLI cli = new CLI();cli.exec(args);}
}

原理很简单：ServiceLocator加载了所有Command的实现类，然后根据子命令调用相应的Command类。AutoService的好处就是不必自己去创建META-INF/services下的相关文件。

测试一下

    @Testpublic void testCreateCommand() {CLI cli = new CLI();String[] argv = { "create",  "-u", "http://www.sina.com.cn"};cli.exec(argv);}@Testpublic void testFetchCommand() {CLI cli = new CLI();String[] argv = { "fetch",  "-u", "http://www.csdn.cn"};cli.exec(argv);}

参考文档

• https://gist.github.com/mkarg/9d9ca23e6da32b47c7fadaf10ae16ba6
• https://pedrorijo.com/blog/java-service-loader/