public class Main {
    public static void main(String[] args) throws Exception{
        BeanInfo beanInfo = Introspector.getBeanInfo(People.class);
        PropertyDescriptor[] propertyDescriptors = beanInfo.getPropertyDescriptors();
        for (PropertyDescriptor propertyDescriptor : propertyDescriptors) {
            System.out.print(propertyDescriptor.getName()+"   ");
        }
    }
    // 程序输出：age   class   name 
    // 为什么会输出class呢？前文中有提到，“看到getName()，内省就会认为这个类中有name字段，但事实上并不一定会有name”，我们知道每个对象都会有getClass方法，所以使用内省时，默认就认为它具有class这个字段
}
class People{
    String name;
    int age;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
}

// 这个类我只保留一些关键的代码，其余的琐碎代码都不看了
public class BeanWrapperImpl extends AbstractNestablePropertyAccessor implements BeanWrapper {
  // 缓存内省的结果，BeanWrapperImpl就是通过这个对象来完成对包装的Bean的属性的控制
  @Nullable
  private CachedIntrospectionResults cachedIntrospectionResults;
    ......       
    public void setBeanInstance(Object object) {
    this.wrappedObject = object;
    this.rootObject = object;
        // 实际进行类型转换的对象：typeConverterDelegate
    this.typeConverterDelegate = new TypeConverterDelegate(this, this.wrappedObject);
    setIntrospectionClass(object.getClass());
  }
    ......
    // 最终调用的就是CachedIntrospectionResults的forClass方法进行内省并缓存，底层调用的就是java的内省机制
    private CachedIntrospectionResults getCachedIntrospectionResults() {
        if (this.cachedIntrospectionResults == null) {
            this.cachedIntrospectionResults = CachedIntrospectionResults.forClass(getWrappedClass());
        }
        return this.cachedIntrospectionResults;
    }
   .......
       // 最终进行类型转换的方法
       private Object convertIfNecessary(@Nullable String propertyName, @Nullable Object oldValue,
                                         @Nullable Object newValue, @Nullable Class<?> requiredType, @Nullable TypeDescriptor td)
       throws TypeMismatchException {
       Assert.state(this.typeConverterDelegate != null, "No TypeConverterDelegate");
       try {
           // 可以看到，最后就是调用typeConverterDelegate来进行类型转换
           return this.typeConverterDelegate.convertIfNecessary(propertyName, oldValue, newValue, requiredType, td);
       }
       ......
   }
}

public abstract class TypeConverterSupport extends PropertyEditorRegistrySupport implements TypeConverter {
      @Nullable
  TypeConverterDelegate typeConverterDelegate;
......
}

public abstract class AbstractPropertyAccessor extends TypeConverterSupport implements ConfigurablePropertyAccessor {
  // 省略部分代码......
  @Override
  public void setPropertyValues(PropertyValues pvs, boolean ignoreUnknown, boolean ignoreInvalid)
      throws BeansException {
    List<PropertyAccessException> propertyAccessExceptions = null;
    List<PropertyValue> propertyValues = (pvs instanceof MutablePropertyValues ?
        ((MutablePropertyValues) pvs).getPropertyValueList() : Arrays.asList(pvs.getPropertyValues()));
    for (PropertyValue pv : propertyValues) {
      try {
        setPropertyValue(pv);
      }
      // ....
    }
  }
  @Override
  @Nullable
  public abstract Object getPropertyValue(String propertyName) throws BeansException;
  @Override
  public abstract void setPropertyValue(String propertyName, @Nullable Object value) throws BeansException;
}