I am new to Selenium and StackOverflow.
I apologize in advance if there is any mistake.
My question is:
How @FindBy() annotation works internally and why initElemets() method is needed?
The method initElements() initializes all of WebElement and List<WebElement> fields with Java Reflection API. It basically changes the default null values to implementations of the interface using Proxy. It also provides sort of Lazy instantiation of the WebElement which means - WebElements are found (looked for?) only when needed - when you invoke operations on them.
In more depth:
initElements accepts Object or Class<?> as an argument and creates an object from it. Having that, PageFactory gets all fields in the class, verifies if any of them is a type of WebElement or List and initializes them.
If WebElement is NOT annotated with @FindBy then PageFactory tries to create selector by the variable name. Something like this WebElement username might actually work without annotation.
If WebElement is annotated with @FindBy then By object is created and connected to the WebElement for later use. This part is just an assumption.
Then, the instance of WebElement is created. Since it's an interface, it cannot be instantiated directly with Reflection API. Proxy is used to handle the creation of a WebElement. PageFactory initializes elements in provided Page Object with newly created instances of WebElements using Reflection.
Both, WebDriver and WebElement implements interface SearchContext which also tells the WebDriver how to look for the element. Is it in the whole page source or scope is reduced to the particular WebElement. That's why we can search elements through other elements like this: element.findElement(By.id("username")).
Hope I explained it a little bit!