看Node源码学Node模块加载

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看Node源码学Node模块加载

↓↓↓↓该符号为断点进入的地方

// a.js
module.exports = {
    str: 'hello'
}

// b.js
const a = ↓↓↓↓require('./a'); // 在这打断点

console.log(a)

然后vscode断点调试

// require方法
  function require(path) {
    try {
      exports.requireDepth += 1;
      return  mod.require(path); ↓↓↓↓
    } finally {
      exports.requireDepth -= 1;
    }
  }

Module.prototype.require = function(path) {
  assert(path, 'missing path');
  assert(typeof path === 'string', 'path must be a string');
  return Module._load(path, this, /* isMain */ false);↓↓↓↓
};

// 该模块有没有缓存，有缓存返回缓存，没有创建一个新的模块，并且load该模块
Module._load = function(request, parent, isMain) {
  if (parent) {
    debug('Module._load REQUEST %s parent: %s', request, parent.id);
  }

  if (isMain && experimentalModules) {

  }

  var filename = Module._resolveFilename(request, parent, isMain);

  var cachedModule = Module._cache[filename];
  if (cachedModule) {
    updateChildren(parent, cachedModule, true);
    return cachedModule.exports;
  }

  if (NativeModule.nonInternalExists(filename)) {
    debug('load native module %s', request);
    return NativeModule.require(filename);
  }

  // Don't call updateChildren(), Module constructor already does.
  var module = new Module(filename, parent);

  if (isMain) {
    process.mainModule = module;
    module.id = '.';
  }

  Module._cache[filename] = module;

  tryModuleLoad(module, filename);↓↓↓↓

  return module.exports;
};

// load文件
function tryModuleLoad(module, filename) {
  var threw = true;
  try {
    module.load(filename);↓↓↓↓
    threw = false;
  } finally {
    if (threw) {
      delete Module._cache[filename];
    }
  }
}

// 执行不同后缀的文件解析，当前用的.js文件加载方法
Module.prototype.load = function(filename) {
  // ...
  var extension = path.extname(filename) || '.js';
  if (!Module._extensions[extension]) extension = '.js';
  Module._extensions[extension](this, filename);↓↓↓↓
  // ....
};

// Native extension for .js
// 读取文件，去除BOM
Module._extensions['.js'] = function(module, filename) {
  var content = fs.readFileSync(filename, 'utf8');
  module._compile(internalModule.stripBOM(content), filename);↓↓↓↓
};

// `_compile`的作用就是调用`vm.runInThisContext`虚拟机，来执行包起来的字符串代码
Module.prototype._compile = function(content, filename) {

  content = internalModule.stripShebang(content);

  // create wrapper function
  var wrapper = Module.wrap(content);↓↓↓↓

  var compiledWrapper = vm.runInThisContext(wrapper, {
    filename: filename,
    lineOffset: 0,
    displayErrors: true
  });
  
  //...

  // 执行编译好的javascript代码
  result = compiledWrapper.call(this.exports, this.exports, require, this, filename, dirname);
  if (depth === 0) stat.cache = null;
  return result;
};

把读取的文件源码，用function包起来`.

  // ...
  Module.wrapper = NativeModule.wrapper;
  Module.wrap = NativeModule.wrap;
  // ...
  NativeModule.wrap = function(script) {
    return NativeModule.wrapper[0] + script + NativeModule.wrapper[1];
  };

  NativeModule.wrapper = [
    '(function (exports, require, module, __filename, __dirname) { ',
    '\n});'
  ];

最后看一下module.js的api

(function (exports, require, module, __filename, __dirname) {

function Module(id, parent) {
  this.id = id;
  this.exports = {};
  this.parent = parent;
  updateChildren(parent, this, false);
  this.filename = null;
  this.loaded = false;
  this.children = [];
}

Module.builtinModules = builtinModules;

Module._cache = Object.create(null);
Module._pathCache = Object.create(null);
Module._extensions = Object.create(null);

Module.wrapper = NativeModule.wrapper;
Module.wrap = NativeModule.wrap;
Module._debug = util.debuglog('module');
Module._findPath = function(request, paths, isMain) {};
Module._nodeModulePaths = function(from) {}
Module._resolveLookupPaths = function(request, parent, newReturn) {};
Module._load = function(request, parent, isMain) {};
Module._resolveFilename = function(request, parent, isMain, options) {};
Module.prototype.load = function(filename) {};
Module.prototype.require = function(path) {};
Module.prototype._compile = function(content, filename) {};


// Native extension for .js
Module._extensions['.js'] = function(module, filename) {};
// Native extension for .json
Module._extensions['.json'] = function(module, filename) {};
//Native extension for .node
Module._extensions['.node'] = function(module, filename) {};


// bootstrap main module.
Module.runMain = function() {};
Module._initPaths = function() {};
Module._preloadModules = function(requests) {};

Module._initPaths();
// backwards compatibility
Module.Module = Module;

});

总结：

加载一个自己写的模块，原理其实很简单，就是:

1. 判断该模块有没有cache,有返回cache
2. 没有，创建一个新的模块 new Module()
3. 读取该模块，得到一个字符串
4. 调用vm.runInThisContext(code[, options])进行编译，
5. 执行返回结果