生成代码的代码 之 POJO生成器

我们在写Java代码时候，有时候需要写一些POJO类，也就是只有一些属性和get, set方法的类。例如，在写REST 服务时候，利用Jersery + Jackson，可以把输入的JSON字符串自动转换成一个POJO类，也可以自动把POJO类转换成JSON字符串而输出。在这样的情况下，我们往往需要写很多的POJO类，里面包含多个属性和对应的get, set方法，写起来比较麻烦。那么，我们有办法能很快的写出这些POJO类吗。

实际上，这些POJO类的实质就是一个类名和几个属性的类型和名称。既然如此，我们可以只提供这几个信息，而通过一个程序来把这些POJO类生成出来。所以，我们要做的是一个生成代码的代码，用来生成POJO对象。

要做这件事情，可以用各种编程语言来实现。笔者个人偏好JavaScript，而且JavaScript里面的object model比较适合做这件事，因此这里笔者通过Node.js来实现。代码如下，回头会把代码开源到GitHub.

用法为， 把POJO的定义写到schema.js文件中，然后运行node createpojo.js

// createpojo.js
var fs = require('fs');
var schema = require("./schema")

var defaultindent = 2;

var getIndent = function(i) {
  return getIndentCore(i, 0);
};
var getIndentCore = function(len, i) {
  if (i >= len) {
    return "";
  }
  return getIndentCore(len, i+1) + " ";
};

var createField = function(field, indentsize) {
  var i = indentsize == undefined ? 2 : indentsize;
  return getIndent(i) + "private " + field.type + " " + field.name + ";";
};

var createFields = function(fields, indentsize) {
  var fieldsstring = fields.map(function(f) {
    return createField(f, indentsize);
  });
  return fieldsstring.join("\n");
};

var createEmptyConstructor = function(clz, indentsize) {
  return getIndent(indentsize) + "public " + clz + "() {\n\n" + getIndent(indentsize) + "}";
};
var createVariable = function(f) {
  return f.type + " " + f.name;
};
var createArglist = function(fields) {
  var args = fields.map(createVariable);
  return args.join(", ");
};
var createAssign = function(field, indentsize) {
  return getIndent(indentsize) + "this." + field.name + " = " + field.name + ";";
};
var createAssigns = function(fields, indentsize) {
  var assigns = fields.map(function(f) {
    return createAssign(f, indentsize);
  });
  return assigns.join("\n");
};
var createConstructor = function(clz, fields, indentsize) {
  return getIndent(indentsize) + "public " + clz + "(" + createArglist(fields) + ") {\n" + createAssigns(fields, indentsize + defaultindent) + "\n" + getIndent(indentsize) + "}";
};
var createConstructors = function(clz, fields, indentsize) {
  return createEmptyConstructor(clz, indentsize) + "\n\n" + createConstructor(clz, fields, indentsize);
};
var getFuncName = function(name) {
  return name.substring(0,1).toUpperCase() + name.substring(1);
};
var createSetter = function(field, indentsize) {
  var line1 = getIndent(indentsize) + "public void set" + getFuncName(field.name) + "(" + createVariable(field) + ") {";
  var line2 = createAssign(field, indentsize + defaultindent);
  var line3 = getIndent(indentsize) + "}";
  var lines = [line1, line2, line3];
  return lines.join("\n");
};
var createGetter = function(field, indentsize) {
  var line1 = getIndent(indentsize) + "public " + field.type + " get" + getFuncName(field.name) + "() {";
  var line2 = getIndent(indentsize + defaultindent) + "return this." + field.name + ";";
  var line3 = getIndent(indentsize) + "}";
  var lines = [line1, line2, line3];
  return lines.join("\n");
};
var createAccessor = function(field, indentsize) {
  var access = [createSetter(field, indentsize), createGetter(field, indentsize)];
  return access.join("\n");
};
var createAccessors = function(fields, indentsize) {
  var accesses = fields.map(function(f) {return createAccessor(f, indentsize);});
  return accesses.join("\n\n");
};
var createClass = function(pkg, clz, fields) {
  var line1 = "package " + pkg + ";";
  var line2 = "public class " + clz + " {";
  var line3 = createFields(fields, defaultindent);
  var line4 = createConstructors(clz, fields, defaultindent);
  var line5 = createAccessors(fields, defaultindent);
  var line6 = "}";
  var lines = [line1, line2, line3, line4, line5, line6];
  return lines.join("\n\n");
};

var classcontents = schema.map(function(s) {
  return createClass(s["package"], s["name"], s["fields"]);
});

for (var i = 0; i < schema.length; i++) {
  fs.writeFile(schema[i].name + ".java", classcontents[i], function(e){
    if (e) {
      console.log("error: " + e);
    }
  });
}

// schema.js
var schemas = [
  {
    "name": "TestClass",
    "package": "example.com",
    "fields": [
      {"name": "name", "type": "String"},
      {"name": "value", "type": "String"}
    ]
  }
]

module.exports = schemas;

代码只是用来做工具用，所以没有写的很完善。如果读者觉得代码有改进之处，欢迎在代码开源之后提交更新。