Spring 是如何解析泛型 - ResolvalbeType

Spring 是如何解析泛型 - ResolvalbeType

Spring 系列目录(https://www.cnblogs.com/binarylei/p/10198698.html)

Java Type 泛型系列文章：

1. Java - Type 介绍
2. Java - Type 的获取方式
3. Spring - ResolvableType

Spring 中大量使用反射，需要获取泛型的具体类型，为此专门提供了一个工具类解析泛型 - ResolvalbeType。ResolvableType 是对 Class，Field，Method 获取 Type 的抽象。

一、ResolvalbeType 使用

interface Service {
}
class ServiceImpl implements Service {
    public ServiceImpl(List> list, Map> map) {
    }
}

(1) forClass

@Test
public void forClassTest() {
    ResolvableType resolvableType = ResolvableType.forClass(ServiceImpl.class);
    // getType 保存原始的 Type 类型
    Assert.assertEquals(ServiceImpl.class, resolvableType.getType());
    // resolve 将 Type 解析为 Class， 如果无法解析返回 null
    Assert.assertEquals(ServiceImpl.class, resolvableType.resolve());
}

(2) forField

private Service service;
private List> list;
private Map> map;
private List[] array;

@Test
public void forFieldTest() {
    // 1. Service service
    Field filed = ReflectionUtils.findField(ResolveTypeTest.class, "service");
    ResolvableType resolvableType = ResolvableType.forField(filed);
    // getType() 保存原始的 Type 类型
    Assert.assertEquals(filed.getGenericType(), resolvableType.getType());
    // resolve() 对于 ParameterizedType 类型保存的是 <> 之前的类型，即 Service.class
    Assert.assertEquals(((ParameterizedType) filed.getGenericType()).getRawType(), resolvableType.resolve());
    
    Class clazz = resolvableType.getGeneric(0).resolve();
    Assert.assertEquals(Double.class, clazz);

    // 2. List> list
    resolvableType = ResolvableType.forField(
            ReflectionUtils.findField(ResolveTypeTest.class, "list"));
    // 下面两种获取泛型的方式等价
    clazz = resolvableType.getGeneric(0).getGeneric(0).resolve();
    Assert.assertEquals(String.class, clazz);    
    clazz = resolvableType.getGeneric(0, 0).resolve();
    Assert.assertEquals(String.class, clazz);

    // 3. Map> map
    resolvableType = ResolvableType.forField(
            ReflectionUtils.findField(ResolveTypeTest.class, "map"));
    clazz = resolvableType.getGeneric(1).getGeneric(1).resolve();
    Assert.assertEquals(Integer.class, clazz);

    // 4. List[] array
    resolvableType = ResolvableType.forField(
            ReflectionUtils.findField(ResolveTypeTest.class, "array"));
    Assert.assertTrue(resolvableType.isArray());
    Assert.assertEquals(List.class, resolvableType.getComponentType().resolve());
    Assert.assertEquals(String.class, resolvableType.getComponentType().getGeneric(0).resolve());
}

(3) forMethodParameter

forMethodParameter 还有很多变种：如 forConstructorParameter、forMethodReturnType

@Test
public void forMethodTest() {
    // 1. 方法的返回值类型
    ResolvableType returnType = ResolvableType.forMethodReturnType(
            ReflectionUtils.findMethod(ServiceImpl.class, "method"));
    Assert.assertEquals(Double.class, returnType.getGeneric(1, 0).resolve());

    // 2. 构造器 ServiceImpl(List> list, Map> map)
    ResolvableType parameterType = ResolvableType.forConstructorParameter(
            ClassUtils.getConstructorIfAvailable(ServiceImpl.class, List.class, Map.class), 0);
    // List> 的泛型第一层为 >，第二层为 
    Assert.assertEquals(String.class, parameterType.getGeneric(0, 0).resolve());

    parameterType = ResolvableType.forConstructorParameter(
            ClassUtils.getConstructorIfAvailable(ServiceImpl.class, List.class, Map.class), 1);
    Assert.assertEquals(Double.class, parameterType.getGeneric(0).resolve());
    Assert.assertEquals(Float.class, parameterType.getGeneric(1, 0).resolve());
    Assert.assertEquals(Integer.class, parameterType.getGeneric(1, 1).resolve());
}

(4) 其它常用方法

@Test
public void test() {
    // HashMap extends AbstractMap implements Map, Cloneable, Serializable
    ResolvableType resolvableType = ResolvableType.forClass(HashMap.class);
    // 1. getInterfaces 获取接口
    Assert.assertEquals(Map.class, resolvableType.getInterfaces()[0].resolve());

    // 2. getSuperType 获取父类
    Assert.assertEquals(AbstractMap.class, resolvableType.getSuperType().resolve());

    // 3. as 向上转型 Map
    ResolvableType mapResolvableType = resolvableType.as(Map.class);
    Assert.assertEquals(Map.class, mapResolvableType.resolve());
    // 4. getRawClass 当 type 是 ParameterizedType 时有效
    Assert.assertEquals(Map.class, mapResolvableType.getRawClass());
    Assert.assertEquals(HashMap.class.getGenericInterfaces()[0], mapResolvableType.getType());

    // 5. getGeneric 获取泛型 class ServiceImpl implements Service
    resolvableType = ResolvableType.forClass(ServiceImpl.class);
    // 当 Type 无法找到具体的 class 类型时返回 null
    Assert.assertEquals("A", resolvableType.getGeneric(0).getType().getTypeName());
    Assert.assertEquals(null, resolvableType.getGeneric(0).resolve());
    // 以下两种获取泛型的 Class 类型方式等价
    Assert.assertEquals(String.class, resolvableType.as(Service.class).getGeneric(0).resolve());
    Assert.assertEquals(String.class, resolvableType.as(Service.class).resolveGeneric(0));

    // 5. getComponentType 获取数组泛型 List[] array
    resolvableType = ResolvableType.forField(
            ReflectionUtils.findField(ResolveTypeTest.class, "array"));
    Assert.assertEquals(List.class, resolvableType.getComponentType().resolve());
}

(5) 创建 ResolvableType

@Test
public void test() {
    ResolvableType resolvableType1 = ResolvableType.forClassWithGenerics(List.class, String.class);
    ResolvableType resolvableType2 = ResolvableType.forArrayComponent(resolvableType1);
    resolvableType2.getComponentType().getGeneric(0).resolve();

    // List[] array
    ResolvableType resolvableType3 = ResolvableType.forField(
            ReflectionUtils.findField(ResolveTypeTest.class, "array"));
    Assert.assertTrue(resolvableType3.isAssignableFrom(resolvableType2));

    Assert.assertTrue(ResolvableType.forClass(Object.class).isAssignableFrom(
            ResolvableType.forClass(String.class)));
}

通过使用也可以看出 ResolvableType 最主要的目的是解析传入的 Type 类型，并通过 resolve() 获取真实的 Class 类型。

二、ResolvalbeType 源码分析

(1) ResolvalbeType 重要属性

// 需要解析的 JDK Type 类型
private final Type type;
// 缓存解析后的 Class 类型
private Class resolved;

// 缓存解析后父类、接口、泛型、数组泛型等
private volatile ResolvableType superType;
private volatile ResolvableType[] interfaces;
private volatile ResolvableType[] generics;
private final ResolvableType componentType;

另外还有两个工具类辅助解析用：

// 对 Type 进行封装
private final TypeProvider typeProvider;
// 对 TypeVariable 如何解析为 Class 的策略
private final VariableResolver variableResolver;

(2) forField

下面以 forField(Field field) 为例，看 ResolvalbeType 是如何解析泛型的。

public static ResolvableType forField(Field field) {
    Assert.notNull(field, "Field must not be null");
    return forType(null, new FieldTypeProvider(field), null);
}

static ResolvableType forType(Type type, TypeProvider typeProvider, VariableResolver variableResolver) {
    if (type == null && typeProvider != null) {
        type = SerializableTypeWrapper.forTypeProvider(typeProvider);
    }
    if (type == null) {
        return NONE;
    }

    // 1. Class 不用解析，所以没必要不缓存
    if (type instanceof Class) {
        return new ResolvableType(type, typeProvider, variableResolver, (ResolvableType) null);
    }
    cache.purgeUnreferencedEntries();

    // 2. 其余的 Type 类型需要解析，所以先缓存起来
    // 2.1 这个构造器专为缓存使用，不会触发 resolveClass() 操作
    ResolvableType resultType = new ResolvableType(type, typeProvider, variableResolver);
    ResolvableType cachedType = cache.get(resultType);
    if (cachedType == null) {
        // 2.2 如果缓存中没有就需要解析了，这个构造器会触发 resolveClass() 操作
        cachedType = new ResolvableType(type, typeProvider, variableResolver, resultType.hash);
        cache.put(cachedType, cachedType);
    }
    resultType.resolved = cachedType.resolved;
    return resultType;
}

解析 Type 对应的真实 Class 类型在 cachedType = new ResolvableType(type, typeProvider, variableResolver, resultType.hash) 这一步，会触发 resolveClass() 操作。

private ResolvableType(Type type, @Nullable TypeProvider typeProvider,
        @Nullable VariableResolver variableResolver, @Nullable Integer hash) {
    this.type = type;
    this.typeProvider = typeProvider;
    this.variableResolver = variableResolver;
    this.componentType = null;
    this.hash = hash;
    this.resolved = resolveClass(); // 关键
}

(3) resolveClass

resolveClass() 方法先对两种简单的类型 Class 和 GenericArrayType 进行了解析，其余的 Type 类型则委托给了 resolveType() 方法解析。

private Class resolveClass() {
    if (this.type == EmptyType.INSTANCE) {
        return null;
    }
    // 1. Class 类型
    if (this.type instanceof Class) {
        return (Class) this.type;
    }
    // 2. GenericArrayType 泛型数组，成员变量的 Class 类型
    if (this.type instanceof GenericArrayType) {
        Class resolvedComponent = getComponentType().resolve();
        return (resolvedComponent != null ? Array.newInstance(resolvedComponent, 0).getClass() : null);
    }
    return resolveType().resolve();
}

对于 Class 和 GenericArrayType 两种 Type 类型：

• Class 解析后仍为该 Class 类型
• GenericArrayType 该泛型数组成员变量的 Class 类型

(4) resolveType

剩余的 ParameterizedType、WildcardType、TypeVariable 三种类型继续解析。

ResolvableType resolveType() {
    // 3. ParameterizedType 类型，getRawType 的 Class 类型
    if (this.type instanceof ParameterizedType) {
        return forType(((ParameterizedType) this.type).getRawType(), this.variableResolver);
    }
    // 4. WildcardType 类型，上界或下界的 Class 类型，如有多个只取第一个
    if (this.type instanceof WildcardType) {
        Type resolved = resolveBounds(((WildcardType) this.type).getUpperBounds());
        if (resolved == null) {
            resolved = resolveBounds(((WildcardType) this.type).getLowerBounds());
        }
        return forType(resolved, this.variableResolver);
    }
    // 5. TypeVariable 类型
    if (this.type instanceof TypeVariable) {
        TypeVariable variable = (TypeVariable) this.type;
        // 5.1 使用 resolveVariable 解析 Try default variable resolution
        if (this.variableResolver != null) {
            ResolvableType resolved = this.variableResolver.resolveVariable(variable);
            if (resolved != null) {
                return resolved;
            }
        }
        // 5.2 TypeVariable 默认取上界的 Class 类型，如有多个只取第一个
        return forType(resolveBounds(variable.getBounds()), this.variableResolver);
    }
    return NONE;
}

resolveType对于 ParameterizedType、WildcardType、TypeVariable 三种 Type 类型：

• ParameterizedType getRawType 对应的 Class 类型
• WildcardType 上界或下界的 Class 类型，如有多个只取第一个
• TypeVariable 默认取上界的 Class 类型，如有多个只取第一个，也可以是 variableResolver 解析，默认为 DefaultVariableResolver

(5) getGenerics

public ResolvableType[] getGenerics() {
    if (this == NONE) {
        return EMPTY_TYPES_ARRAY;
    }
    ResolvableType[] generics = this.generics;
    if (generics == null) {
        // 1. 获取 Class 类型的泛型
        if (this.type instanceof Class) {
            Type[] typeParams = ((Class) this.type).getTypeParameters();
            generics = new ResolvableType[typeParams.length];
            for (int i = 0; i < generics.length; i++) {
                generics[i] = ResolvableType.forType(typeParams[i], this);
            }
        // 2. 获取 ParameterizedType 类型的泛型
        } else if (this.type instanceof ParameterizedType) {
            Type[] actualTypeArguments = ((ParameterizedType) this.type).getActualTypeArguments();
            generics = new ResolvableType[actualTypeArguments.length];
            for (int i = 0; i < actualTypeArguments.length; i++) {
                generics[i] = forType(actualTypeArguments[i], this.variableResolver);
            }
        // 3. WildcardType、TypeVariable 类型调用 resolveType() 重新解析
        } else {
            generics = resolveType().getGenerics();
        }
        this.generics = generics;
    }
    return generics;
}

参考：

1. 《Spring ResolvableType 更好的处理泛型》：https://blog.csdn.net/u012881904/article/details/80813294

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