The sentence sounds fine, but is a bit unclear. When using "which" to introduce a relative clause, place a comma before it if the clause is non-restrictive, meaning it could be removed and the sentence would still make sense. Also, when using non-restrictive clauses, the clause usually refers to the noun directly preceding it. In the above example, "which" does not refer to "web development." It could refer to either "Java," or "programming language." You could say, "In web development, as Java is a thorough programming language, which has a lot of mature frameworks and specifications, I really want to develop my career in Java programming." Now the sentence is more clear, yet still a bit clunky. It would be best to replace "which" with "that," as a way to avoid the commas without having to rewrite the sentence. This way the sentence would read, "As Java is a thorough programming language in web development that has a lot of mature frameworks and specifications, I really want to develop my career in Java programming."