how much should i know to became java developer.

Erm, enough to develop using Java?

You may have to be a little more explicit in your question for us to answer in any meaningful way I'm afraid...

Java is HUGE in scope - I doubt that any Java developer ever knows all its aspects.

Have a look at Oracle's Java tutorials - in my opinion the best and only ones worth working with. The overview is here. You definitly need to be familiar with all the stuff in the "Basics" trail, the JDBC database access section, and most probably either the Swing or JavaFX GUI section. After that it depends on what applications you will be writing. Note that this is just Java SE - the core language and APIs. After that there's Java EE ("Enterprise Edition") that adds lots of extra stuff, most notably for writing large-scale corporate web server database apps. There's also Java for Android which is very similar to Java SE with some extra stuff to interface with the Android OS in the way Android expects.

So, that's an overview of the whole thing. I hope you realise now that you will need to start with a small subset of all that, and that which subset you chose depends entirely on what kind of application you will be writing. If you can clarify that then people here can help you focus in the right areas.

Way too broad. What level developer? Development is far more than just knowing the syntax of a language (or many languages, hardly anyone works exclusively in a single language).

There's design, architecture, performance, domain knowledge, database administration, testing, and yes, some sysadmin and management skills.
As a senior dev I spend as much time analysing problems, talking with customers and suppliers, and discussing ideas and plans with team members and managers as I do actually programming and testing the system I'm working on.

Becoming a Java developer is not easy, just like all other languages. You have to learn a lot. I also was a beginner but I have evolved a lot now. I first started with Oracle tutorials, then I buy good books like those from the apress site. But I still have a lot to learn I admit. My goal is to be able to realize an application myself without the help of anyone, be it for Android, Windows or iOS. I advise you to first address the basic concepts of Java SE. This is what I did and now I can do Windows applications in Java. With this base already, targeting other OS will be very easy. And for modern UIs, focus on JavaFx, replacing Swing and Awt.

Becoming a java programmer or developer is not easy.. but there are many ways u can learn through tutorial sites, youtube videos etc..