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200 行 Java 代码实现依赖注入框架

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)