Re: GCJ and C++ (calling Java from C++)

[Lothar Werzinger <lothar at tradescape dot biz>]

>From what I learned reading about CNI calling methods on interfaces requires a 
special dispatch table called otable. I checked the Library and the 
LibraryTest class variables in the debugger and found that the Library class 
has otable = 0x0 which seems to be the culprit of the 
java.lang.NullPointerException when calling a method.

Has anybody a clue why the otable for that class is NULL?

Here's a snippet of code that executes without problems:
    jclass LibraryTestClass = &LibraryTest::class$;
    JvInitClass(LibraryTestClass);
    System::out->print(JvNewStringUTF("LibraryTest::class$.getName()        = 
"));
    System::out->println(LibraryTestClass->getName());
    System::out->print(JvNewStringUTF("p_libraryTest->getClass()->getName() = 
"));
    System::out->println(p_libraryTest->getClass()->getName());

    Library * p_library = p_libraryTest->getlibrary();
    cout << "p_library=" << hex << p_library << endl;
    
    jclass LibraryClass = &Library::class$;
    JvInitClass(LibraryClass);
    System::out->print(JvNewStringUTF("Library::class$.getName()            = 
"));
    System::out->println(LibraryClass->getName());
    System::out->print(JvNewStringUTF("p_library->getClass()->getName()     = 
"));
    System::out->println(p_library->getClass()->getName());

the failure occurs here:
    String * p_name = p_library->getName();
    System::out->println(p_name);


Here's the gdb output:
(gdb) p *LibraryClass
$1 = {<> = {<No data fields>}, static class$ = {<> = {<No data fields>}, 
static class$ = <same as static member of an already seen type>,
    next_or_version = 0x5626ff40, name = 0x56045276, accflags = 17, superclass 
= 0x562f1de0, constants = {size = 8, tags = 0x562e8728 "", data = 
0x562f5480},
    methods = 0x562f2080, method_count = 53, vtable_method_count = 41, fields 
= 0x0, size_in_bytes = 140, field_count = 0, static_field_count = 0,
    vtable = 0x562f24c8, otable = 0x0, otable_syms = 0x0, atable = 0x0, 
atable_syms = 0x0, itable = 0x0, itable_syms = 0x0, catch_classes = 
0x562e8710,
    interfaces = 0x562f206c, loader = 0x0, interface_count = 1, state = 14 
'\016', thread = 0x1, depth = 1, ancestors = 0x804f0e0, idt = 0x804f0f0,
    arrayclass = 0x454c0, protectionDomain = 0x0, assertion_table = 0x0, 
hack_signers = 0x0, chain = 0x562efc40, aux_info = 0x0, engine = 0x5647006c},
  next_or_version = 0x80061a80, name = 0x555c70fa, accflags = 1537, superclass 
= 0x0, constants = {size = 3, tags = 0x555d48a4 "", data = 0x555d649c},
  methods = 0x555d63c0, method_count = 11, vtable_method_count = -1, fields = 
0x0, size_in_bytes = 4, field_count = 0, static_field_count = 0, vtable = 
0x0,
  otable = 0x0, otable_syms = 0x0, atable = 0x0, atable_syms = 0x0, itable = 
0x0, itable_syms = 0x0, catch_classes = 0x555d48a8, interfaces = 0x555d48a0,
  loader = 0x0, interface_count = 1, state = 7 '\a', thread = 0x1e7e88, depth 
= 0, ancestors = 0x0, idt = 0x0, arrayclass = 0x0, protectionDomain = 0x0,
  assertion_table = 0x0, hack_signers = 0x0, chain = 0x0, aux_info = 0x0, 
engine = 0x0}
(gdb) p *LibraryTestClass
$2 = {<> = {<No data fields>}, static class$ = {<> = {<No data fields>}, 
static class$ = <same as static member of an already seen type>,
    next_or_version = 0x5626ff40, name = 0x56045276, accflags = 17, superclass 
= 0x562f1de0, constants = {size = 8, tags = 0x562e8728 "", data = 
0x562f5480},
    methods = 0x562f2080, method_count = 53, vtable_method_count = 41, fields 
= 0x0, size_in_bytes = 140, field_count = 0, static_field_count = 0,
    vtable = 0x562f24c8, otable = 0x0, otable_syms = 0x0, atable = 0x0, 
atable_syms = 0x0, itable = 0x0, itable_syms = 0x0, catch_classes = 
0x562e8710,
    interfaces = 0x562f206c, loader = 0x0, interface_count = 1, state = 14 
'\016', thread = 0x1, depth = 1, ancestors = 0x804f0e0, idt = 0x804f0f0,
    arrayclass = 0x454c0, protectionDomain = 0x0, assertion_table = 0x0, 
hack_signers = 0x0, chain = 0x562efc40, aux_info = 0x0, engine = 0x5647006c},
  next_or_version = 0x555e04e0, name = 0x804bf56, accflags = 33, superclass = 
0x562f1de0, constants = {size = 43, tags = 0x804d360 "", data = 0x804d2a0},
  methods = 0x804d200, method_count = 7, vtable_method_count = 8, fields = 
0x804d1c0, size_in_bytes = 20, field_count = 4, static_field_count = 0,
  vtable = 0x1e4d98, otable = 0x804d960, otable_syms = 0x804d38c, atable = 
0x804d9a0, atable_syms = 0x804d434, itable = 0x804da00, itable_syms = 
0x804d500,
  catch_classes = 0x804d5c0, interfaces = 0x0, loader = 0x0, interface_count = 
0, state = 14 '\016', thread = 0x1e7e89, depth = 1, ancestors = 0x8067fb8,
  idt = 0x56224880, arrayclass = 0x0, protectionDomain = 0x0, assertion_table 
= 0x804d5e0, hack_signers = 0x0, chain = 0x56335660, aux_info = 0x0,
  engine = 0x5647006c}

Lothar
-- 
Lothar Werzinger Dipl.-Ing. Univ.
framework & platform architect
Tradescape Inc.
111 West St. John Street, Suite 200
San Jose, Ca 95113
email: lothar@tradescape.biz