CODEWITHSUNDEEP
javaunsupported-class-version
Marc Klein
Marc Klein

Reputation: 13

Why can Java 8 load Java 9 class?

I have following setting:

Class A - compiled with java 9 (class version 53)
Class B - Should be compiled with java 8 (class version 52)

Class B has a dependency to Class A.

Now I get a surprising result: I can successfully compile Class B with Java 8 although it has a dependency to the java 9 compiled class A.

If I compile Class A with Java 10 or Java 11 and then try to compile class B with Java 8 I get the expected error message:

class file has wrong version 55.0 (or 54.0), should be 52.0

So can anybody explain why I don't get the message

class file has wrong version 53.0, should be 52.0

if I try to use Java 8 with Java 9 classes?

I use this java versions:

Java 8: Oracle jdk 1.8.0_144 (installed in folder d:\jdk8)
Java 9: Oracle jdk 9.0.1 (installed in folder d:\jdk9)
Java 11: Oracle jdk 11.0.1 (installed in folder d:\jdk11)

To demonstrate it I created this two class files:

File A.java:

public class A {
    public void test() {
        System.out.println("Class A");
    }
}

File B.java:

public class B {
    public static void main(String[] args) {
        A classA = new A();
        classA.test();

        System.out.println("Class B");
    }
}

First A.java is compiled with Java 9:
d:\jdk9\bin>javac d:\test\A.java

Then B.java is compiled with Java 8 using the Class A that is compiled with Java 9:
d:\jdk8\bin>javac -cp d:\test d:\test\B.java

warning: d:\test\A.class: major version 53 is newer than 52, the highest major version supported by this compiler.
It is recommended that the compiler be upgraded.
1 warning

So the class file is compiled successfully with a warning. I just saw this warning by invoking the compiler manually. If I use Maven to build the project, this warning wasn't visible.

Now Class A is compiled with Java 11 (same with Java 10):
d:\jdk11\bin>javac d:\test\A.java

If I try to compile Class B, I get an error:

d:\jdk8\bin>javac -cp d:\test d:\test\B.java

d:\test\B.java:3: error: cannot access A
A classA = new A();
^
bad class file: d:\test\A.class
class file has wrong version 55.0, should be 52.0
Please remove or make sure it appears in the correct subdirectory of the classpath.
1 error

So it seems that technically it's possible to use Java 9 compiled class files with Java 8. But I did not think it was possible.

Upvotes: 0

Views: 689

Answers (1)

Joop Eggen
Joop Eggen

Reputation: 109557

The java 9 compiler javac probably compiled for java 8; one can use a newer compiler to compile for an older .class version. Look for -target and -source 1.8.

Either the IDE or maven/gradle was configured for an older version.

Upvotes: 1

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