Java 11 面试题与答案

问题 1. What is Java 11?

Java 11 is the second release of the long-term support (LTS) after Java 8. Since Java 11, Oracle JDK is no longer free for commercial use. You can use it in the development stages, but you need to buy a license to use it commercially. After Java 11, Oracle will not provide free long-term (LTS) support for any single Java version.

问题 2. What is the difference between Oracle JDK and OpenJDK?

To check the detail difference, please check here: OracleJDK vs OpenJDK 💯💯

• Ever since Java 7, Java has been developed out in the open in the OpenJDK project. It's open source, but it's mainly driven by committers from Oracle, the owners of Java. But there are also many outside contributions from the likes of Red Hat, Twitter, and IBM. When you want to run Java, you have a choice to either use the OpenJDK releases or to use the Oracle JDK release. The Oracle JDK release used to be a slightly extended and amended version of OpenJDK. The ultimate goal here is to have no differences between the Oracle JDK builds and the OpenJDK builds.
• The main difference between Oracle JDK and OpenJDK is that Oracle JDK incorporates some commercial features. Rather than stripping these features to get conversions between Oracle JDK and OpenJDK, Oracle has committed to open source these commercial features into OpenJDK. This process already started with Java 9 and 10 and is continued with Java 11.
• With Java 11, the goal of a convergence between the Oracle JDK and OpenJDK code bases have been achieved. there's some pretty big differences in licensing. OpenJDK is GPL 2 licensed, so it has a true open source license. Oracle JDK, on the other hand, has a proprietary license called the Oracle Binary Code License Agreement. Up until Java 10, you could use both OpenJDK and Oracle JDK in production free of charge. For Oracle JDK, you could buy optional support from Oracle. That's changing with Java 11. For OpenJDK there are no changes, you can still use the GPL 2 license builds. However, you can no longer use Oracle JDK free of charge in production.

问题 3. Which commercial features are available as open source in Java 11?

One of the Oracle JDK commercial features that have been open-sourced is Java Flight Recorder. In addition to Java Flight Recorder, Java Mission Control has also been open sourced.

问题 4. What is Flight Recorder in Java 11?

Java Flight Recorder is an always-on, low-overhead, data collection framework that you can use to get metrics on your JVMs. It's implemented as a bounded circular buffer, which buffers internal JVM metrics for a configurable amount of minutes. The great thing about this feature is that you can leave it enabled on your prediction systems because it's so low overhead.

问题 5. What is Java Mission Control in Java 11?

Java Mission Control is an application that can analyze the dumps that come from Java Flight Recorder, and give you a graphical overview of what's happening inside of a JVM.