Existing State

Implementation of most Java editing features needs a model of Java source code (like ASTs). The model that is currently used is the model provided by javac(star). NetBeans is in fact using a modified fork of the
JDK's javac. In the fork the signatures of the APIs are unmodified, although the behavior and internals are sometimes tweaked. NetBeans itself is providing another layer of APIs that enhance the javac ones.

The Java editing features (like code completion, formatting, errors, etc.) are mostly written against the (javac or NetBeans) API only, although they sometimes expect the modified behavior (e.g. specific spans for AST nodes, especially for input with compile-time errors). Some of the core modules (java.source.base, java.source, spi.java.hints, etc.) are also using javac internals.

The usage is direct - there is no intermediate API, the Java editing features call directly methods on the model API. There are NetBeans APIs that provide access to the AST, but walking though the AST, etc., is mostly done using the javac APIs directly.

(star) The Java compiler in JDK. The relevant javac APIs are javax.lang.model.**, javax.tools.** (although parts of this API are supersedded by NetBeans APIs), javax.annotation.processing.** and com.sun.source.**.

Using javac from the JDK

In the jdk-javac branch, the NetBeans internals are changed so that they can run with unmodified javac from the runtime JDK. This requires running on JDK 9 or later, as it depends on model APIs added in JDK 9. nb-javac can be installed, in which can it would be used instead of the runtime javac. This should mostly lead to the existing state, except for bugs and very few exceptions.

Advantages

Caveats

When running without nb-javac, there are several caveats. A probably incomplete list:

The above can be (except for bugs) resolved by installing nb-javac, or it may be possible to partially mitigate some of the problems. A tricky problem in this branch is:

void test(Map<? extends String, ? extends Number> map) {
        map.entrySet();//assign return value to a new variable here
    }

Which will produce incorrect results (on the jdk-javac branch, with or without nb-javac).

Additional caveats: