依赖注入框架并不神秘, 其实它是非常简单的东西. 不要去看 spring 的依赖注入源码, 因为你只要一去看就意味着你再也写不敢下手自己撸了, 它的功能因为过于强大, 所以设计也过于复杂, 普通程序员一眼看去只能望洋兴叹.
我也并没有去细致阅读 spring 源码. 即便如此也只用了半天的时间便自己撸了一个基本满足标准依赖注入规范JSR-330的小框架 iockids. 这个小框架只有一个主类 Injector, 大约 200 行代码, 它具备以下功能.
单例 / 非单例注入
构造器注入
字段注入
循环依赖注入
Qualifier 注入
我们看一个稍微复杂一点的使用示例
- import javax.inject.Inject;
- import javax.inject.Named;
- import javax.inject.Singleton;
- import iockids.Injector;
- @Singleton
- class Root {
- @Inject
- @Named("a")
- Node a;
- @Inject
- @Named("b")
- Node b;
- @Override
- public String toString() {
- return String.format("root(%s, %s)", a.name(), b.name());
- }
- }
- interface Node {
- String name();
- }
- @Singleton
- @Named("a")
- class NodeA implements Node {
- @Inject
- Leaf leaf;
- @Inject
- @Named("b")
- Node b;
- @Override
- public String name() {
- if (b == null)
- return String.format("nodeA(%s)", leaf);
- else
- return String.format("nodeAWithB(%s)", leaf);
- }
- }
- @Singleton
- @Named("b")
- class NodeB implements Node {
- Leaf leaf;
- @Inject
- @Named("a")
- Node a;
- @Inject
- public NodeB(Leaf leaf) {
- this.leaf = leaf;
- }
- @Override
- public String name() {
- if (a == null)
- return String.format("nodeB(%s)", leaf);
- else
- return String.format("nodeBWithA(%s)", leaf);
- }
- }
- class Leaf {
- @Inject
- Root root;
- int index;
- static int sequence;
- public Leaf() {
- index = sequence++;
- }
- public String toString() {
- if (root == null)
- return "leaf" + index;
- else
- return "leafwithroot" + index;
- }
- }
- public class Demo {
- public static void main(String[] args) {
- var injector = new Injector();
- injector.registerQualifiedClass(Node.class, NodeA.class);
- injector.registerQualifiedClass(Node.class, NodeB.class);
- var root = injector.getInstance(Root.class);
- System.out.println(root);
- }
- }
上面这份代码用到了 iockids 提供的所有功能.
Root/NodeA/NodeB 类是单例类
Leaf 类是非单例类
它们都使用了字段注入
NodeB 使用了构造器注入
NodeA 和 NodeB 还使用了 Qualifier 名称注入
Leaf 类中有 Root 类型的字段, 这便是循环依赖
NodeA 中有 NodeB 字段, NodeB 中有 NodeA 字段, 这也是循环依赖
为了便于理解上述代码, 我画了依赖图
上面的代码输出如下
root(nodeAWithB(leafwithroot0), nodeBWithA(leafwithroot1))
从这个输出中, 我们也可以大致想象出依赖结构.
iockids 提供了丰富的注入错误异常报告, 防止用户注入配置出错.
比如我们将上面的 NodeA 和 NodeB 的名称都配置成一样的 a, 就会曝出下面的错误堆栈
iockids.InjectException: duplicated qualifier javax.inject.Named with the same class iockids.demo.Node
- at iockids.Injector.registerQualifiedClass(Injector.java:87)
- at iockids.Injector.registerQualifiedClass(Injector.java:70)
- at iockids.demo.Demo.main(Demo.java:106)
如果我们将 NodeB 的构造器随意加一个参数
- @Inject
- public NodeB(Leaf leaf, int k) {
- this.leaf = leaf;
- }
运行时就会抛出下面的错误
- iockids.InjectException: no accessible constructor for injection class int
- at iockids.Injector.createNew(Injector.java:120)
- at iockids.Injector.createNew(Injector.java:94)
- at iockids.Injector.createFromParameter(Injector.java:167)
- at iockids.Injector.createFromConstructor(Injector.java:145)
- at iockids.Injector.createNew(Injector.java:123)
- at iockids.Injector.createFromQualified(Injector.java:216)
- at iockids.Injector.createFromField(Injector.java:173)
- at iockids.Injector.injectMembers(Injector.java:233)
- at iockids.Injector.createNew(Injector.java:136)
- at iockids.Injector.createFromQualified(Injector.java:216)
- at iockids.Injector.createFromField(Injector.java:173)
- at iockids.Injector.injectMembers(Injector.java:233)
- at iockids.Injector.createNew(Injector.java:136)
- at iockids.Injector.createNew(Injector.java:94)
- at iockids.Injector.getInstance(Injector.java:245)
- at iockids.demo.Demo.main(Demo.java:107)
项目开源地址: https://github.com/pyloque/iockids
来源: http://www.tuicool.com/articles/buIr22