Annotations

• Annotations help us in determining basic errors before hand before compilation.
• For example if we misspell an overridden method then in that case we can check it before compilation only if it has @override annotation above it.
• In Spring framework many issues can be predicted in advanced i.e. before application compiler.
• Before annotations we used to do all configurations in Spring framework in the xml configuration file.
• for example @bean/@component annotation directly class can be used as a bean. 
• We can create a custom annotation using @interface MyAnnotation {}
• Important meta annotation properties are
• @Target
• This annotation meta tells where we can apply the particular annotation on method on class or constructor, et cetera
• It has following properties
• ElementType.Constructor
• ElementType.Method
• ElementType.Parameter
• ElementType.Field
• @Retention
• This annotation meta Tells how the particular annotation will be stored in Java.
• It has following values
• RetentionPolicy.Source
• Annotations will be discarded by compiler itself and not even recorded in class file.
• RetentionPolicy.Class
• Annotation will be recorded in class file, but ignored by JVM during runtime.
• RetentionPolicy.Runtime
• Annotation will be recorded in class file and also available during runtime.
• Return type Of the custom annotation is Restricted to
• Primitive type(type,boolean,double)
• String
• Enum
• Class<?>
• Annotation
• Array of above types.