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activiti源码分析:设计模式

  • 时间:2016-09-28
  • 发布:昆明Java培训
  • 来源:达内新闻

昆明Java培训机构的老师明白对activiti有基本了解的朋友都知道,activiti暴露了七个接口来提供工作流的相关服务,这些接口具体是如何实现的呢?查看源码发现其实现的形式大体如下:

public class RuntimeServiceImpl extends ServiceImpl implements RuntimeService {

public ProcessInstance startProcessInstanceByKey(String processDefinitionKey) {

return commandExecutor.execute(new StartProcessInstanceCmd<ProcessInstance>(processDefinitionKey, null, ull, null));

}

public ProcessInstance startProcessInstanceByKey(String processDefinitionKey, String businessKey) {

return commandExecutor.execute(new StartProcessInstanceCmd<ProcessInstance>(processDefinitionKey, null, businessKey, null));

}

...

}

service中的大部分方法都是通过调用commandExecutor.execute()完成的,然而点进去看则会发现什么都没有:

public class CommandExecutorImpl implements CommandExecutor {

private final CommandConfig defaultConfig;

private final CommandInterceptor first;

public CommandExecutorImpl(CommandConfig defaultConfig, CommandInterceptor first) {

this.defaultConfig = defaultConfig;

this.first = first;

}

public CommandInterceptor getFirst() {

return first;

}

@Override

public CommandConfig getDefaultConfig() {

return defaultConfig;

}

@Override

public <T> T execute(Command<T> command) {

return execute(defaultConfig, command);

}

@Override

public <T> T execute(CommandConfig config, Command<T> command) {

return first.execute(config, command);

}

}

看到这里就会发现并不能看出这条语句究竟做了什么,那么究竟是如何提供服务的呢?其实activiti中大部分操作都是基于设计模式中的命令模式完成的(这里还使用了职责链模式,构造了命令拦截器链,用于在命令真正被执行之前做一系列操作)。下面昆明Java培训机构的老师结合源码详细介绍一下这些设计思路:

命令模式的本质在于将命令进行封装,发出命令和执行命令分离。职责链模式只需要将请求放入职责链上,其处理细节和传递都不需要考虑。activiti将这两个模式整合在一起,构成了其服务主要的实现方式。其核心只有三个部分:CommandExecutor(命令执行器,用于执行命令),CommandInterceptor(命令拦截器,用于构建拦截器链),Command(命令自身)。这三个接口是整个核心的部分,还会涉及到其它的关键类,之后会一一说明,这三个类都在activiti-engine.jar这个activiti实现的核心包下,具体位置是:org.activiti.engine.impl.interceptor。下面昆明Java培训机构的老师由这三个接口逐步介绍相关的类和具体实现:

三个接口源码:

public interface Command <T> {

T execute(CommandContext commandContext);

}

/**

* The command executor for internal usage.

*/

public interface CommandExecutor {

/**

* @return the default {@link CommandConfig}, used if none is provided.

*/

CommandConfig getDefaultConfig();

/**

* Execute a command with the specified {@link CommandConfig}.

*/

<T> T execute(CommandConfig config, Command<T> command);

/**

* Execute a command with the default {@link CommandConfig}.

*/

<T> T execute(Command<T> command);

}

public interface CommandInterceptor {

<T> T execute(CommandConfig config, Command<T> command);

CommandInterceptor getNext();

void setNext(CommandInterceptor next);

}

Command的接口中只有一个execute方法,这里才是写命令的具体实现,而CommandExecutor的实现类在上面已经给出,其包含了一个CommandConfig和一个命令拦截器CommandInterceptor,而执行的execute(command)方法,实际上调用的就是commandInterceptor.execute(commandConfig,command)。CommandInterceptor中包含了一个set和get方法,用于设置next(实际上就是下一个CommandInterceptor)变量。想象一下,这样就能够通过这种形式找到拦截器链的下一个拦截器链,就可以将命令传递下去。

简单梳理一下:Service实现服务的其中一个标准方法是在具体服务中调用commandExecutor.execute(new command())(这里的command是具体的命令)。其执行步骤就是命令执行器commandExecutor.execute调用了其内部变量CommandInterceptor first(第一个命令拦截器)的execute方法(加上了参数commandConfig)。CommandInterceptor类中包含了一个CommandInterceptor对象next,用于指向下一个CommandInterceptor,在拦截器的execute方法中,只需要完成其对应的相关操作,然后执行一下next.execute(commandConfig,command),就可以很简单的将命令传递给下一个命令拦截器,然后在最后一个拦截器中执行command.execute(),调用这个命令最终要实现的内容就行了。

实现一个自定义的命令只需要实现Command<T>接口,在execute中做相应的操作就行了,而实现一个自定义的命令拦截器需要继承AbstractCommandInterceptor,在execute中做相应的处理,最后调用next.execute()即可,而命令执行器虽然也可以自己实现,但是没有多大意义,非常麻烦。前面说过,命令执行器会先执行命令拦截器链的execute方法,但命令拦截器链是如何构建的,命令又是在哪里调用的,第一个拦截器是如何添加到命令执行器的,这些都要关注于Activiti工作流引擎的初始化。

初始化的方法主要写在了org.activiti.engine.impl.cfg.ProcessEngineConfigurationImpl类的init()方法中,这里主要关注于其中的initCommandExecutors(),如果对activiti的配置不清楚的,可以好好的了解一下这个初始化过程。

initCommandExecutors():

protected void initCommandExecutors() {

initDefaultCommandConfig();

initSchemaCommandConfig();

initCommandInvoker();

initCommandInterceptors();

initCommandExecutor();

}

这五个方法名很清楚地说明了初始化步骤,前两步都是初始化CommandConfig,第一个就是命令执行器的defaultConfig,主要用在transaction拦截器。第三步初始化命令执行者,这也是一个拦截器,不过其放在拦截器的尾端,最后一个执行,它的execute方法就是调用了command.execute()。第四步就是初始化命令拦截器了。最后一步初始化命令执行器。

前三步相关的类:

/**

* CommandConfig实际就这两个配置

*/

public class CommandConfig {

private boolean contextReusePossible;

private TransactionPropagation propagation;

// DefaultConfig

public CommandConfig() {

this.contextReusePossible = true;

this.propagation = TransactionPropagation.REQUIRED;

}

// SchemaCommandConfig

public CommandConfig transactionNotSupported() {

CommandConfig config = new CommandConfig();

config.contextReusePossible = false;

config.propagation = TransactionPropagation.NOT_SUPPORTED;

return config;

}

}

CommandInvoker:

public class CommandInvoker extends AbstractCommandInterceptor {

@Override

public <T> T execute(CommandConfig config, Command<T> command) {

return command.execute(Context.getCommandContext());

}

@Override

public CommandInterceptor getNext() {

return null;

}

@Override

public void setNext(CommandInterceptor next) {

throw new UnsupportedOperationException("CommandInvoker must be the last interceptor in the chain");

}

}

接下来看看关键的第四步:

protected void initCommandInterceptors() {

if (commandInterceptors==null) {

commandInterceptors = new ArrayList<CommandInterceptor>();

if (customPreCommandInterceptors!=null) {

commandInterceptors.addAll(customPreCommandInterceptors);

}

commandInterceptors.addAll(getDefaultCommandInterceptors());

if (customPostCommandInterceptors!=null) {

commandInterceptors.addAll(customPostCommandInterceptors);

}

commandInterceptors.add(commandInvoker);

}

}

protected Collection< ? extends CommandInterceptor> getDefaultCommandInterceptors() {

List<CommandInterceptor> interceptors = new ArrayList<CommandInterceptor>();

interceptors.add(new LogInterceptor());

CommandInterceptor transactionInterceptor = createTransactionInterceptor();

if (transactionInterceptor != null) {

interceptors.add(transactionInterceptor);

}

interceptors.add(new CommandContextInterceptor(commandContextFactory, this));

return interceptors;

}

这段代码可以看出,activiti提供了默认的命令拦截器,其顺序是LogInterceptor->TransactionInterceptor->CommandContextInterceptor,也能看出activiti提供了配置自定义的拦截器可能,customPreCommandInterceptors和customPostCommandInterceptors,只需要set进入配置就行了。一个在默认拦截器之前,一个在之后,最后一个添加的就是commandInvoker。最终的命令拦截器链就是customPreCommandInterceptors->LogInterceptor->TransactionInterceptor->CommandContextInterceptor->customPostCommandInterceptors->commandInvoker。

最后一步初始化命令执行器代码包括了构建拦截器链:

protected void initCommandExecutor() {

if (commandExecutor==null) {

CommandInterceptor first = initInterceptorChain(commandInterceptors);

commandExecutor = new CommandExecutorImpl(getDefaultCommandConfig(), first);

}

}

protected CommandInterceptor initInterceptorChain(List<CommandInterceptor> chain) {

if (chain==null || chain.isEmpty()) {

throw new ActivitiException("invalid command interceptor chain configuration: "+chain);

}

for (int i = 0; i < chain.size()-1; i++) {

chain.get(i).setNext( chain.get(i+1) );

}

return chain.get(0);

}

最后我们看一看默认提供的三个拦截器都做了一些什么操作(不包括最后CommandInvoker,上面已给出)。

LogInterceptor.execute():

if (!log.isDebugEnabled()) {

// do nothing here if we cannot log

return next.execute(config, command);

}

log.debug("\n");

log.debug("--- starting {} --------------------------------------------------------", command.getClass().getSimpleName());

try {

return next.execute(config, command);

} finally {

log.debug("--- {} finished --------------------------------------------------------", command.getClass().getSimpleName());

log.debug("\n");

}

TransactionInterceptor.execute()(这是一个抽象的方法,需要自己实现,下面以与spring集成后所给的实现为例)

protected CommandInterceptor createTransactionInterceptor() {

if (transactionManager == null) {

throw new ActivitiException("transactionManager is required property for SpringProcessEngineConfiguration, use "

+ StandaloneProcessEngineConfiguration.class.getName() + " otherwise");

}

return new SpringTransactionInterceptor(transactionManager);

}

SpringTransactionInterceptor:

public class SpringTransactionInterceptor extends AbstractCommandInterceptor {

private static final Logger LOGGER = LoggerFactory.getLogger(SpringTransactionInterceptor.class);

protected PlatformTransactionManager transactionManager;

public SpringTransactionInterceptor(PlatformTransactionManager transactionManager) {

this.transactionManager = transactionManager;

}

public <T> T execute(final CommandConfig config, final Command<T> command) {

LOGGER.debug("Running command with propagation {}", config.getTransactionPropagation());

TransactionTemplate transactionTemplate = new TransactionTemplate(transactionManager);

transactionTemplate.setPropagationBehavior(getPropagation(config));

T result = transactionTemplate.execute(new TransactionCallback<T>() {

public T doInTransaction(TransactionStatus status) {

return next.execute(config, command);

}

});

return result;

}

private int getPropagation(CommandConfig config) {

switch (config.getTransactionPropagation()) {

case NOT_SUPPORTED:

return TransactionTemplate.PROPAGATION_NOT_SUPPORTED;

case REQUIRED:

return TransactionTemplate.PROPAGATION_REQUIRED;

case REQUIRES_NEW:

return TransactionTemplate.PROPAGATION_REQUIRES_NEW;

default:

throw new ActivitiIllegalArgumentException("Unsupported transaction propagation: " + config.getTransactionPropagation());

}

}

}

最后一个CommandContextInterceptor.execute():

public <T> T execute(CommandConfig config, Command<T> command) {

CommandContext context = Context.getCommandContext();

boolean contextReused = false;

// We need to check the exception, because the transaction can be in a rollback state,

// and some other command is being fired to compensate (eg. decrementing job retries)

if (!config.isContextReusePossible() || context == null || context.getException() != null) {

context = commandContextFactory.createCommandContext(command);    

}

else {

log.debug("Valid context found. Reusing it for the current command '{}'", command.getClass().getCanonicalName());

contextReused = true;

}

try {

// Push on stack

Context.setCommandContext(context);

Context.setProcessEngineConfiguration(processEngineConfiguration);

return next.execute(config, command);

} catch (Exception e) {

context.exception(e);

} finally {

try {

if (!contextReused) {

context.close();

}

} finally {

// Pop from stack

Context.removeCommandContext();

Context.removeProcessEngineConfiguration();

Context.removeBpmnOverrideContext();

}

}

return null;

}

这里昆明Java培训机构的老师提醒大家值得注意的是context.close()方法,这里将调用session.flush();,真正执行完成数据库操作。Context也是一个比较重要的类,有兴趣可以研究一下。

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