In Java programs, every instance of an inner class has an immediately enclosing instance. When an inner class is instantiated with the `new` keyword, the immediately enclosing instance is passed to the inner class's constructor via a parameter not visible in source code. The Java Language Specification (§15.9.4) requires that the immediately enclosing instance is not null.

Prior to JDK 25, `javac` inserted a null check on the immediately enclosing instance before it was passed to the inner class's constructor. No null check was performed in the body of the inner class's constructor. As a result, it was possible to use core reflection, e.g., `java.lang.reflect.Constructor::newInstance`, or specially crafted `class` files to invoke an inner class's constructor with null as the immediately enclosing instance. This could lead to unexpected program failures.

In JDK 25, `javac` inserts an additional null check on the immediately enclosing instance. This check occurs in the body of the inner class's constructor. As a result, there is no way to instantiate an inner class with null as the immediately enclosing instance.

In the extremely rare case of needing to run legacy code that passes null as the immediately enclosing instance, developers can, at their own risk, use the unsupported option `-XDnullCheckOuterThis=false`. This prevents `javac` from inserting the additional null check in inner class constructor bodies.