Java generating a parameterless constructor when you don't have any other is like a polite waiter taking your coat for you. And, this is logically correct since, if you want to block creation of objects without any data in it, this is one good way.įor example, consider that an employee object must have an employee id associated to it.įor achieving this, define a single argument constructor and don't define no-argument constructor. So it will give an error if you call no-argument constructor after defining a parameterised constructor, since it finds no no-argument constructor explicitly defined in class. It's just like authenticating any other method of a class. When you do not explicitly write a no-argument constructor for a class, the compiler won't complain as long as objects are built without parameter constructors.(since compiler allows default constructor to create objects, which itself gives call to no-argument constructor).īut if you do define a no-argument constructor, when compiling, compiler will check for the call to constructor and its definition in class. (" -> PrivateInaccessible.First of all, default constructor is not generated, its provided by compiler if no-argument constructor is not written explicitly.
Javac -source 1.8 -target 1.8 CallDefaultMethodThroughReflection.java It’s also available in this gist here.Ĭompile it with JDK 9 or 10 (because it also tries using JDK 9+ API: MethodHandles.privateLookupIn()), but compile it using this command, so you can also run the class on JDK 8:
The following class tries all combinations. So far, I’ve just been trying to run different things on different JDKs. So, there’s a solution (hack) that works on Java 8 but not on 9 or 10, and there’s a solution that works on Java 9 and 10, but not on Java 8. Great, it works in Java 9 and 10, what about Java 8?Ĭaused by: : no private access for invokespecial: interface Duck, from ProxyDemoĪt $Lookup.findSpecial(MethodHandles.java:1002) One of the Jigsaw project’s goals is to precisely not allow such hacks to persist. If we run the program with the -illegal-access=deny flag:Įxception in thread "main" : Unable to make $Lookup() accessible: module java.base does not "opens " to unnamed module java.base/.checkCanSetAccessible(AccessibleObject.java:337)Īt java.base/.checkCanSetAccessible(AccessibleObject.java:281)Īt java.base/.checkCanSetAccessible(Constructor.java:192)Īt java.base/.setAccessible(Constructor.java:185) WARNING: All illegal access operations will be denied in a future release WARNING: Use -illegal-access=warn to enable warnings of further illegal reflective access operations WARNING: Please consider reporting this to the maintainers of ProxyDemo WARNING: Illegal reflective access by ProxyDemo (file:/C:/Users/lukas/workspace/playground/target/classes/) to constructor $Lookup() WARNING: An illegal reflective access operation has occurred When googling further, we might find the following solution, which accesses MethodHandles.Lookup‘s internals through reflection: We get the following IllegalAccessException:Ĭaused by: : no private access for invokespecial: interface Duck, from Duck/packageĪt .makeAccessException(MemberName.java:850)Īt $Lookup.checkSpecialCaller(MethodHandles.java:1572)Īt $Lookup.unreflectSpecial(MethodHandles.java:1231)īummer.
#Java reflection get default constructor code
The almost same code snippet no longer works. So, the original Google search turned up results that indicate we need to use the MethodHandles API. You simply cannot do this):Įxception in thread "main" Ĭaused by: Īt 0(Native Method)Īt (NativeMethodAccessorImpl.java:62)Īt (DelegatingMethodAccessorImpl.java:43)Īt .invoke(Method.java:498)Ĭaused by: Thread.currentThread().getContextClassLoader(),īut this will just generate a long long stack trace of nested exceptions (this isn’t specific to the method being a default method. Duck duck = (Duck) Proxy.newProxyInstance(
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