Class C
package bar;
import foo.D;
public class C {
public static void main(String[] arguments) {
Class k = D.class;
System.out.println(k);
}
}
Class D is public when Class C is compiled.
package foo;
public class D {
}
Executing bar.C shows "class foo.D".
suztomo-macbookpro44% mvn clean package
...
suztomo-macbookpro44% mv target/linkage-checker-test-1.0-SNAPSHOT.jar d-public.jar
suztomo-macbookpro44% java -cp target/classes:d-public.jar bar.C
class foo.D
We change class D to be non-public:
package foo;
class D {
}
(corresponding lines in class C are commented out)
suztomo-macbookpro44% mvn clean compile
...
# We don't need this version of class bar.C.
suztomo-macbookpro44% rm -rf target/classes/bar
Then run the class C and class. I get IllegalAccessError just to touch the class literal.
suztomo-macbookpro44% java -cp target/classes:d-public.jar bar.C
Exception in thread "main" java.lang.IllegalAccessError: tried to access class foo.D from class bar.C
at bar.C.main(C.java:27)
suztomo-macbookpro44% javap -private -verbose -cp d-public.jar bar.C
Classfile jar:file:/Users/suztomo/wsgenoptions/d-public.jar!/bar/C.class
Last modified Sep 15, 2020; size 553 bytes
MD5 checksum c7adc0c202c8e66d94db0748bdc7f551
Compiled from "C.java"
public class bar.C
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #6.#22 // java/lang/Object."<init>":()V
#2 = Class #23 // foo/D
#3 = Fieldref #24.#25 // java/lang/System.out:Ljava/io/PrintStream;
#4 = Methodref #26.#27 // java/io/PrintStream.println:(Ljava/lang/Object;)V
#5 = Class #28 // bar/C
#6 = Class #29 // java/lang/Object
#7 = Utf8 <init>
#8 = Utf8 ()V
#9 = Utf8 Code
#10 = Utf8 LineNumberTable
#11 = Utf8 LocalVariableTable
#12 = Utf8 this
#13 = Utf8 Lbar/C;
#14 = Utf8 main
#15 = Utf8 ([Ljava/lang/String;)V
#16 = Utf8 arguments
#17 = Utf8 [Ljava/lang/String;
#18 = Utf8 k
#19 = Utf8 Ljava/lang/Class;
#20 = Utf8 SourceFile
#21 = Utf8 C.java
#22 = NameAndType #7:#8 // "<init>":()V
#23 = Utf8 foo/D
#24 = Class #30 // java/lang/System
#25 = NameAndType #31:#32 // out:Ljava/io/PrintStream;
#26 = Class #33 // java/io/PrintStream
#27 = NameAndType #34:#35 // println:(Ljava/lang/Object;)V
#28 = Utf8 bar/C
#29 = Utf8 java/lang/Object
#30 = Utf8 java/lang/System
#31 = Utf8 out
#32 = Utf8 Ljava/io/PrintStream;
#33 = Utf8 java/io/PrintStream
#34 = Utf8 println
#35 = Utf8 (Ljava/lang/Object;)V
{
public bar.C();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 25: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lbar/C;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=2, args_size=1
0: ldc #2 // class foo/D
2: astore_1
3: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;
6: aload_1
7: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
10: return
LineNumberTable:
line 27: 0
line 28: 3
line 29: 10
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 arguments [Ljava/lang/String;
3 8 1 k Ljava/lang/Class;
}
SourceFile: "C.java"
The bytecode above has 0: ldc #2 // class foo/D to load the class D, which we change non-public at runtime.
This causes the IllegalAccessError as documented in https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-6.html#jvms-6.5.ldc.