设计模式-命令模式（Command）

定义:
将一个请求封装成一个对象，从而可以不同的请求对客户进行参数化；对请求排队或记录请求日志，以及支持可撤销的操作。

命令模式结构示意图:

注： 创建具体的命令对象，并且设置命令对象的接受者。注意这个不是我们常规意义上的客户端，而是在组装命令和接受者，或许，把这个client称为装配者更好理解，因为真正使用命令的客户端是从Invoker来触发执行。

Command (recognizeable by behavioral methods in an abstract/interface type which invokes a method in an implementation of a different abstract/interface type which has been encapsulated by the command implementation during its creation)

All implementations of java.lang.Runnable

所有实现Runnable接口的对象都是命令模式结构。

org.activiti.engine.impl.TaskServiceImpl;

class TaskServiceImpl extends ServiceImpl implements TaskService {
    protected CommandExecutor commandExecutor;

    public void claim(String taskId, String userId) {
        this.commandExecutor.execute(new ClaimTaskCmd(taskId, userId));
    }

    public void unclaim(String taskId) {
        this.commandExecutor.execute(new ClaimTaskCmd(taskId, (String)null));
    }

    public void complete(String taskId) {
        this.commandExecutor.execute(new CompleteTaskCmd(taskId, (Map)null));
    }

    public void complete(String taskId, Map<String, Object> variables) {
        this.commandExecutor.execute(new CompleteTaskCmd(taskId, variables));
    }

    public void complete(String taskId, Map<String, Object> variables, boolean localScope) {
        this.commandExecutor.execute(new CompleteTaskCmd(taskId, variables, localScope));
    }
}

public interface CommandExecutor {
    CommandConfig getDefaultConfig();

    <T> T execute(CommandConfig var1, Command<T> var2);

    <T> T execute(Command<T> var1);
}

public interface Command<T> {
    T execute(CommandContext var1);
}

class CompleteTaskCmd extends NeedsActiveTaskCmd<Void> {
    protected Void execute(CommandContext commandContext, TaskEntity task) {
        if(this.variables != null) {
            if(this.localScope) {
                task.setVariablesLocal(this.variables);
            } else if(task.getExecutionId() != null) {
                task.setExecutionVariables(this.variables);
            } else {
                task.setVariables(this.variables);
            }
        }

        task.complete(this.variables, this.localScope);
        return null;
    }
}

#####activiti初始化##########
org.activiti.engine.impl.cfg.ProcessEngineConfigurationImpl.init();

protected void initCommandExecutors() {
    this.initDefaultCommandConfig();
    this.initSchemaCommandConfig();
    this.initCommandInvoker();
    this.initCommandInterceptors();
    this.initCommandExecutor();
}

命令模式调用顺序示意图：