How to solve InaccessibleObjectException ("Unable to make {member} accessible: module {A} does not 'opens {package}' to {B}") on Java 9?

This exception occurs in a wide variety of scenarios when running an application on Java 9. Certain libraries and frameworks (Spring, Hibernate, JAXB) are particularly prone to it. Here's an example from Javassist:

java.lang.reflect.InaccessibleObjectException: Unable to make protected final java.lang.Class java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int,java.security.ProtectionDomain) throws java.lang.ClassFormatError accessible: module java.base does not "opens java.lang" to unnamed module @1941a8ff
    at http://ift.tt/2h9UsTo
    at http://ift.tt/2ifaNUu
    at http://ift.tt/2h9KH7L
    at http://ift.tt/2ifkG4E
    at javassist.util.proxy.SecurityActions.setAccessible(SecurityActions.java:102)
    at javassist.util.proxy.FactoryHelper.toClass2(FactoryHelper.java:180)
    at javassist.util.proxy.FactoryHelper.toClass(FactoryHelper.java:163)
    at javassist.util.proxy.ProxyFactory.createClass3(ProxyFactory.java:501)
    at javassist.util.proxy.ProxyFactory.createClass2(ProxyFactory.java:486)
    at javassist.util.proxy.ProxyFactory.createClass1(ProxyFactory.java:422)
    at javassist.util.proxy.ProxyFactory.createClass(ProxyFactory.java:394)

The message says:

Unable to make protected final java.lang.Class java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int,java.security.ProtectionDomain) throws java.lang.ClassFormatError accessible: module java.base does not "opens java.lang" to unnamed module @1941a8ff

What can be done to avoid the exception and have the program run successfully?