类spring ioc 泛型保留
类spring ioc 泛型保留
什么是泛型擦除
Java并不会传递泛型类，举个直观的栗子:
@Component
public class BaseProvider<T>
{
public void doSomething()
{
Class<?> clazz = getClass();
System.out.println(clazz.getName()+" doSomething");
Type superclass = clazz.getGenericSuperclass();
System.out.println("superclass="+superclass);
}
}
这里doSomething尝试打印泛型类型，
@Component
public class TestService
{
@Autowired
private BaseProvider<User> userProvdier;
public void doSomething()
{
userProvdier.doSomething();
}
}
BaseProvider<User>泛型指定了User类，来个测试看看User是否能被获取到？
@ComponentScan
public class Main
{
public static void main(String[] args)
{
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(Main.class);
TestService service = context.getBean(TestService.class);
System.out.println("service=" + service);
service.doSomething();
}
}
依赖脚本build.gradle
dependencies {
String springVersion="5.1.9.RELEASE"
implementation "org.springframework:spring-context:$springVersion"
}
运行可以看到结果是，spring ioc并不能注入获取泛型
service=TestService@a68df9
BaseProvider doSomething
superclass=class java.lang.Object
自定义IOC泛型注入
在解决spring泛型问题之前，先做一个简单的IOC泛型注入框架，来了解其原理
TestService service = Context.getBean(TestService.class);
}
这里将自写一个Context类，理解为上下文也好，BeanFactory也好，其原理不过是创建管理对象的东西
public class Context
{
public static <T> T getBean(Class<T> clazz)
{
return createBean(clazz, null, null, null);
}
private static <T> T createBean(Class<T> clazz, Type type, Object target, Field source)
{
//...
}
//....
}
设计createBean创建bean对象，依赖于bean类型，泛型类型，上级调用对象，来源位置（字段或方法）。再一个缓存bean对象，基本功能:
private static final Map<String, Object> beanCache = new HashMap<>();
@SuppressWarnings("unchecked")
private static <T> T createBean(Class<T> clazz, Type type, Object target, Member source)
{
try
{
String beanName = getBeanName(clazz, type, target, source);
if (beanCache.containsKey(beanName))
{
return (T) beanCache.get(beanName);
}
if (type != null && type instanceof ParameterizedType)
{
//创建泛型对象代理类
}
Constructor<T> cts = clazz.getDeclaredConstructor();
T obj = cts.newInstance();// 创建object
Field[] fields = clazz.getDeclaredFields();
for (Field field : fields)
{
if (field.getAnnotation(Autowried.class) != null)
{
setField(field, obj, createBean(field.getType(), field.getGenericType(), obj, field));
}
}
beanCache.put(beanName, obj);
return obj;
} catch (Exception e)
{
e.printStackTrace();
}
return null;
}
private static String getBeanName(Class<?> clazz, Type type, Object target, Member source)
{
if (type != null && type instanceof ParameterizedType)
{
ParameterizedType pt = (ParameterizedType) type;
StringJoiner sb = new StringJoiner("_");
for (Type p : pt.getActualTypeArguments())
{
sb.add(p.getTypeName().replaceAll("\\.", "_"));
}
return clazz.getName() + "_$_" + sb.toString() + "_Proxy";
}
return clazz.getName();
}
getBeanName规定了beanname的生成规则，createBean中创建泛型代理的部分，这里使用javassist去生成动态代理类
implementation 'org.javassist:javassist:3.25.0-GA'
生成泛型子类
ClassPool pool = ClassPool.getDefault();
ParameterizedType pt = (ParameterizedType) type;
CtClass subClass = pool.get(clazz.getName());
StringJoiner genericSignature=new StringJoiner(",","L"+getClassPath(subClass.getName())+"<",">;");
Type[] ptts = pt.getActualTypeArguments();
for(int i=0;i<ptts.length;i++) {
genericSignature.add("L"+getClassPath(ptts[i].getTypeName())+";");
}
CtClass proxyClass = pool.makeClass( beanName,subClass);
proxyClass.setGenericSignature( genericSignature.toString());
clazz = (Class<T>) proxyClass.toClass();
getClassPath替换classname为class路径
private static String getClassPath(String name)
{
return name.replaceAll("\\.", "/");
}
再来看看运行效果
service=TestService@5d6f64b1
BaseProvider_$_User_Proxy doSomething
superclass=BaseProvider<User>
泛型<User>能够获取出来
为什么要泛型注入
泛型注入的应用场景，Java获取泛型一直是个很头疼的是，如果能够轻松获取泛型，就能够减少大量的代码。举个栗子，写一个常用的数据库操作工具类，不能获取泛型情况下，就必须传入Class<T> beanClazz参数：
public class DbUtil<T>{
List<T> getAll(Class<T> beanClazz){
//....
}
}
思考
方法的泛型应该如何去获取？
预告
将在下篇文章中讲解如何在spring 中解决泛型问题
