What on earth is java ee?

after reading the answers on the Internet, I still don"t quite understand what javaEE is. I hope a great god can help me answer my questions.

1. What exactly does he contain physically? I looked at the official website of oracle and there was no jdk, in the download list. Why?

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"JAVA EE Platform SDK"

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The

directory structure is like this. What are these for? (for example) how can I use it?

2. Is it true that "javaee is a series of specifications and implementations (providing specifications for servlet and jsp)" is the core of EJB?

3. If 2 is right, where can I see the specification provided?

4. The Internet says: why is javaEE a specification? I really don"t understand this concept, how can a good javaEE become a norm? So javaSE,javaME has become the norm? It doesn"t make sense. You say that javaEE provides specifications and I can understand) isn"t javaEE a "java platform for enterprise complex servers"? Is it a specification or a platform?

after reading a big circle on the Internet, I was still quite confused, so I came here to post an inquiry, which may not be very good. I hope you can solve the doubt. Thank you

Java-ee java
Mar.11,2021

although I have been using Java for more than ten years, I seldom use JavaEE, to explain my superficial understanding:

  1. JavaEE is the specification, it is mainly used to solve all kinds of complex problems in enterprise applications, such as transaction processing, distributed processing, etc. Specifically, you can query on the web page, it is given in the form of a series of JSR, each JSR is a specification, you can download and read, the EJB you mentioned is one of these specifications.
  2. the program that implements these JavaEE specifications is the JavaEE server, which is usually called Application Server,. For example, the GlassFish, you mentioned above is a JavaEE implementation provided by Oracle. If developers follow the JavaEE specification to develop enterprise applications, they can run in these Application Server or migrate between multiple Application Server.

because of the complexity of JavaEE, most Java enterprise applications do not use JavaEE servers. For example, the most popular Tomcat, is not a JavaEE server, but a Servlet server (only a small part of the complete JavaEE specification is implemented).

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