activiti源码学习之命令模式

什么是命令模式？

大都很熟悉的设计模式，此问题不在本文介绍范围内。

activiti中的命令模式

activiti使用命令模式作为基础开发模型。把每个操作都封装为一个命令，降低代码的耦合度，避免臃肿的大类的出现。

activiti是怎样运用的命令模式？

相信看activiti手册的都会看到这张图：

这张图描述了ProcessEngine与XXXservice的关系。而XXXService提供了执行流程的所有方法。XXXService的每个方法都执行对应一个cmd。

如启动流程方法：

RuntimeServiceImpl.startProcessInstanceByKey

 public ProcessInstance startProcessInstanceByKey(String processDefinitionKey, String businessKey, Map<String, Object> variables) {return commandExecutor.execute(new StartProcessInstanceCmd<ProcessInstance>(processDefinitionKey, null, businessKey, variables));}

StartProcessInstanceCmd：就为启动流程的命令，里面为启动流程的相关逻辑代码。

上面是执行的典型例子，下面看是如何执行的Cmd呢？

先看XXXService的结构

commandExecutor：从名字可以看出，命令执行器，它提供一个执行命令的方法execute。

不难看出，commandExecutor是命令执行的关键。

看CommnadExecutor的结构图：

下面将从命令环境的初始化到命令执行完的整个过程看看。

以与Spring结合为例

1。初始化命令执行环境：

在ProcessEngineConfigurationImpl的初始化方法中初始化命令执行环境：

protected void init() {initHistoryLevel();initExpressionManager();initVariableTypes();initBeans();initFormEngines();initFormTypes();initScriptingEngines();initBusinessCalendarManager();initCommandContextFactory();initTransactionContextFactory();initCommandExecutors();initServices();initIdGenerator();initDeployers();initJobExecutor();initMailScanner();initDataSource();initTransactionFactory();initSqlSessionFactory();initSessionFactories();initJpa();initDelegateInterceptor();initEventHandlers();}

protected void initCommandExecutors() {initActualCommandExecutor();initCommandInterceptorsTxRequired();initCommandExecutorTxRequired();initCommandInterceptorsTxRequiresNew();initCommandExecutorTxRequiresNew();}

a. 初始化CommandExecutorImpl

protected void initActualCommandExecutor() {actualCommandExecutor = new CommandExecutorImpl();}

b.初始化命令拦截器

protected void initCommandInterceptorsTxRequired() {if (commandInterceptorsTxRequired==null) {if (customPreCommandInterceptorsTxRequired!=null) {commandInterceptorsTxRequired = new ArrayList<CommandInterceptor>(customPreCommandInterceptorsTxRequired);} else {commandInterceptorsTxRequired = new ArrayList<CommandInterceptor>();}commandInterceptorsTxRequired.addAll(getDefaultCommandInterceptorsTxRequired());if (customPostCommandInterceptorsTxRequired!=null) {commandInterceptorsTxRequired.addAll(customPostCommandInterceptorsTxRequired);}commandInterceptorsTxRequired.add(actualCommandExecutor);}}

看SpringProcessEngineConfiguration实现了ProcessEngineConfigurationImpl的getDefaultCommandInterceptorsTxRequired

将LogInterceptor、SpringTransactionInterceptor和CommandContextInterceptor都添加到命令列表中

protected Collection< ? extends CommandInterceptor> getDefaultCommandInterceptorsTxRequired() {if (transactionManager==null) {throw new ActivitiException("transactionManager is required property for SpringProcessEngineConfiguration, use "+StandaloneProcessEngineConfiguration.class.getName()+" otherwise");}List<CommandInterceptor> defaultCommandInterceptorsTxRequired = new ArrayList<CommandInterceptor>();defaultCommandInterceptorsTxRequired.add(new LogInterceptor());defaultCommandInterceptorsTxRequired.add(new SpringTransactionInterceptor(transactionManager, TransactionTemplate.PROPAGATION_REQUIRED));CommandContextInterceptor commandContextInterceptor = new CommandContextInterceptor(commandContextFactory, this);defaultCommandInterceptorsTxRequired.add(commandContextInterceptor);return defaultCommandInterceptorsTxRequired;}

依次设置拦截器的下一个拦截器是谁，在执行的过程中，按顺序执行的拦截器。这里将LogInterceptor作为第一个拦截器。

protected void initCommandExecutorTxRequired() {if (commandExecutorTxRequired==null) {commandExecutorTxRequired = initInterceptorChain(commandInterceptorsTxRequired);}}

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);}

从以上代码可以看出，一共添加了四个拦截器，分别为：LogInterceptor、SpringTransactionInterceptor、CommandContextInterceptor和CommandExecutorImpl

protected void initCommandInterceptorsTxRequiresNew() {if (commandInterceptorsTxRequiresNew==null) {if (customPreCommandInterceptorsTxRequiresNew!=null) {commandInterceptorsTxRequiresNew = new ArrayList<CommandInterceptor>(customPreCommandInterceptorsTxRequiresNew);} else {commandInterceptorsTxRequiresNew = new ArrayList<CommandInterceptor>();}commandInterceptorsTxRequiresNew.addAll(getDefaultCommandInterceptorsTxRequiresNew());if (customPostCommandInterceptorsTxRequiresNew!=null) {commandInterceptorsTxRequiresNew.addAll(customPostCommandInterceptorsTxRequiresNew);}commandInterceptorsTxRequiresNew.add(actualCommandExecutor);}}

protected void initCommandExecutorTxRequiresNew() {if (commandExecutorTxRequiresNew==null) {commandExecutorTxRequiresNew = initInterceptorChain(commandInterceptorsTxRequiresNew);}}

2.初始化Services,将CommandExecutor对象赋值给Service，这里的CommandExecutor为LogInterceptor。

protected void initServices() {initService(repositoryService);initService(runtimeService);initService(historyService);initService(identityService);initService(taskService);initService(formService);initService(managementService);}protected void initService(Object service) {if (service instanceof ServiceImpl) {((ServiceImpl)service).setCommandExecutor(commandExecutorTxRequired);}}

3.以启动流程为例，看Command的执行过程

a.调用RuntimeServcie的启动流程方法：

public ProcessInstance startProcessInstanceByKey(String processDefinitionKey, Map<String, Object> variables) {return commandExecutor.execute(new StartProcessInstanceCmd<ProcessInstance>(processDefinitionKey, null, null, variables));}

b.从上面初始化过程可知，此时的commandExecutor为LogInterceptor,即执行LogInterceptor的execute方法

public <T> T execute(Command<T> command) {log.fine("                                                                                                    ");log.fine("--- starting " + ClassNameUtil.getClassNameWithoutPackage(command) + " --------------------------------------------------------");try {return next.execute(command);} finally {log.fine("--- " + ClassNameUtil.getClassNameWithoutPackage(command) + " finished --------------------------------------------------------");log.fine("                                                                                                    ");}}

c.从上面初始化过程可知，LogInterceptor的next为SpringTransactionInterceptor,即执行SpringTransactionInterceptor的execute方法(SpringTransactionInterceptor的作用是进行事务管理)

public class SpringTransactionInterceptor extends CommandInterceptor {protected PlatformTransactionManager transactionManager;protected int transactionPropagation;public SpringTransactionInterceptor(PlatformTransactionManager transactionManager, int transactionPropagation) {this.transactionManager = transactionManager;this.transactionPropagation = transactionPropagation;}@SuppressWarnings("unchecked")public <T> T execute(final Command<T> command) {TransactionTemplate transactionTemplate = new TransactionTemplate(transactionManager);transactionTemplate.setPropagationBehavior(transactionPropagation);T result = (T) transactionTemplate.execute(new TransactionCallback() {public Object doInTransaction(TransactionStatus status) {return next.execute(command);}});return result;}
}

d.从上面的初始化过程可知，SpringTransactionInterceptor的next为CommandContextInterceptor,这个CommnadContextInterceptor很关键，系统中的命令上下文就是在这里初始化并传入到Cmd的。命令上下文保存了Cmd执行时不可缺少的如持久化需要的session等重要的对象。非常重要。

public <T> T execute(Command<T> command) {CommandContext context = commandContextFactory.createCommandContext(command);try {Context.setCommandContext(context);Context.setProcessEngineConfiguration(processEngineConfiguration);return next.execute(command);} catch (Exception e) {context.exception(e);} finally {try {context.close();} finally {Context.removeCommandContext();Context.removeProcessEngineConfiguration();}}return null;}

e.从上面的初始化过程可知，CommandContextInterceptor的next为CommnadExecutorImpl

public class CommandExecutorImpl extends CommandInterceptor {public <T> T execute(Command<T> command) {return command.execute(Context.getCommandContext());}
}

e.执行cmd的execute方法

此时执行完毕。

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