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  "title": "同步和异步&阻塞和非阻塞",
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  "contentMarkdown": "# 同步和异步&阻塞和非阻塞\n\n# 同步和异步&阻塞和非阻塞\n\n> 创建时间：2020/6/17 21:05\n\n## 同步和异步\n\n同步和异步关注的是消息通信机制 (synchronous communication/ asynchronous communication)\n\n### 同步（Synchronous）\n\n同步，就是在发出一个 _调用_ 时，在没有得到结果之前，该 _调用_ 就不返回。但是一旦调用返回，就得到返回值了。换句话说，就是由 _调用者_ 主动等待这个 _调用_ 的结果。\n\n> 同步是指：当程序1调用程序2时，程序1停下不动，直到程序2完成回到程序1来，程序1才继续执行下去。\n\n> 同步是指：发送方发出数据后，等接收方发回响应以后才发下一个数据包的通讯方式。\n\n### 异步（Asynchronous）\n\n_调用_ 在发出之后，方法调用就会立即返回，调用者就可以继续后续的操作。换句话说，当一个异步过程调用发出后，调用者不会立刻得到结果。而是在 _调用_ 发出后， _被调用者_ 通过状态、通知来通知调用者，或通过回调函数处理这个调用。\n\n> 异步是指：当程序1调用程序2时，程序1径自继续自己的下一个动作，不受程序2的的影响。\n\n> 异步是指：发送方发出数据后，不等接收方发回响应，接着发送下个数据包的通讯方式。\n\n### 如何选择\n\n就是看需要的请求的数据是否是程序继续执行必须依赖的数据\n优缺点：\n\n  1. 同步的执行效率会比较低，耗费时间，但有利于我们对流程进行控制，避免很多不可掌控的意外情况；\n  2. 异步的执行效率高，节省时间，但是会占用更多的资源，也不利于我们对进程进行控制\n\n## 阻塞和非阻塞\n\n阻塞和非阻塞关注的是程序在等待调用结果（消息，返回值）时的状态.\n\n### 阻塞\n\n阻塞调用是指调用结果返回之前，当前线程会被挂起。调用线程只有在得到结果之后才会返回。\n\n### 非阻塞\n\n非阻塞调用指在不能立刻得到结果之前，该调用不会阻塞当前线程。\n"
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