1 public class Animal{
 2     public void say(){
 3         System.out.println("is animal");
 4     }
 5     public static void main(String[] args){
 6         Animal animal = new Dog();
 7         run(animal);
 8         animal = new Cat();
 9         run(animal);
10     }
11     public static void run(Animal animal){
12         animal.say();
13     }
14 }
15 class Dog extends Animal{
16     public void say(){
17         System.out.println("is Dog");
18     }
19 }
20 class Cat extends Animal{
21     public void say(){
22         System.out.println("is Cat");
23     }
24 }

 1 class Cpuls{
 2 public: 
 3     int x;
 4 public:
 5      void func(){
 6         this->x = 2;
 7     }
 8 };
 9 int main(){
10     Cpuls cplus;
11     return 0;
12 }

void klassVtable::compute_vtable_size_and_num_mirandas(
    int* vtable_length_ret, int* num_new_mirandas,
    GrowableArray<Method*>* all_mirandas, Klass* super,
    Array<Method*>* methods, AccessFlags class_flags,
    Handle classloader, Symbol* classname, Array<Klass*>* local_interfaces,
    TRAPS) {
  No_Safepoint_Verifier nsv;

  // set up default result values
  int vtable_length = 0;

  // start off with super's vtable length
  InstanceKlass* sk = (InstanceKlass*)super;
  //获取父类 vtable 的大小，并将当前类的 vtable 的大小设置为父类 vtable 的大小。
  vtable_length = super == NULL ? 0 : sk->vtable_length();

  // go thru each method in the methods table to see if it needs a new entry
  int len = methods->length();//方法个数
  for (int i = 0; i < len; i++) {
    assert(methods->at(i)->is_method(), "must be a Method*");
    methodHandle mh(THREAD, methods->at(i));

    /*循环遍历当前 Java 类的每一个方法 ，调用 needs_new_vtable_entry()函数进行判断，
    如果判断的结果是 true ，则将 vtable 的大小增 1 */
    if (needs_new_vtable_entry(mh, super, classloader, classname, class_flags, THREAD)) {
      vtable_length += vtableEntry::size(); // we need a new entry
    }
  }

  GrowableArray<Method*> new_mirandas(20);
  // compute the number of mirandas methods that must be added to the end
  get_mirandas(&new_mirandas, all_mirandas, super, methods, NULL, local_interfaces);
  *num_new_mirandas = new_mirandas.length();

  // Interfaces do not need interface methods in their vtables
  // This includes miranda methods and during later processing, default methods
  if (!class_flags.is_interface()) {
    vtable_length += *num_new_mirandas * vtableEntry::size();
  }

  if (Universe::is_bootstrapping() && vtable_length == 0) {
    // array classes don't have their superclass set correctly during
    // bootstrapping
    vtable_length = Universe::base_vtable_size();
  }

  if (super == NULL && !Universe::is_bootstrapping() &&
      vtable_length != Universe::base_vtable_size()) {
    // Someone is attempting to redefine java.lang.Object incorrectly.  The
    // only way this should happen is from
    // SystemDictionary::resolve_from_stream(), which will detect this later
    // and throw a security exception.  So don't assert here to let
    // the exception occur.
    vtable_length = Universe::base_vtable_size();
  }
  assert(super != NULL || vtable_length == Universe::base_vtable_size(),
         "bad vtable size for class Object");
  assert(vtable_length % vtableEntry::size() == 0, "bad vtable length");
  assert(vtable_length >= Universe::base_vtable_size(), "vtable too small");

  *vtable_length_ret = vtable_length;//返回虚方法个数
}

bool klassVtable::needs_new_vtable_entry(methodHandle target_method,
                                         Klass* super,
                                         Handle classloader,
                                         Symbol* classname,
                                         AccessFlags class_flags,
                                         TRAPS) {

/*如果 Java 方法被 final、static修饰，或者 Java 类被 final 修饰，或者 Java 方法是构造
 函数<init>，则返回 false */
  if (class_flags.is_interface()) {
    // Interfaces do not use vtables, except for java.lang.Object methods,
    // so there is no point to assigning
    // a vtable index to any of their local methods.  If we refrain from doing this,
    // we can use Method::_vtable_index to hold the itable index
    return false;
  }

  if (target_method->is_final_method(class_flags) ||
      // a final method never needs a new entry; final methods can be statically
      // resolved and they have to be present in the vtable only if they override
      // a super's method, in which case they re-use its entry
      (target_method()->is_static()) ||
      // static methods don't need to be in vtable
      (target_method()->name() ==  vmSymbols::object_initializer_name())
      // <init> is never called dynamically-bound
      ) {
    return false;
  }

  // Concrete interface methods do not need new entries, they override
  // abstract method entries using default inheritance rules
  if (target_method()->method_holder() != NULL &&
      target_method()->method_holder()->is_interface()  &&
      !target_method()->is_abstract() ) {
    return false;
  }

  // we need a new entry if there is no superclass
  if (super == NULL) {
    return true;
  }

  // private methods in classes always have a new entry in the vtable
  // specification interpretation since classic has
  // private methods not overriding
  // JDK8 adds private  methods in interfaces which require invokespecial
  if (target_method()->is_private()) {
    return true;
  }

  // Package private methods always need a new entry to root their own
  // overriding. This allows transitive overriding to work.
  if (target_method()->is_package_private()) {
    return true;
  }

  // search through the super class hierarchy to see if we need
  // a new entry
  /*遍历父类中同名 、签名也完全相同的方法，如果父类方法的访问权限是 public 或者 protected,
  并且没有 static 或 private 修饰，则说明子类重写了父类的方法，此时返回 false*/
  ResourceMark rm;
  Symbol* name = target_method()->name();//当前方法名
  Symbol* signature = target_method()->signature();
  Klass* k = super;
  Method* super_method = NULL;
  InstanceKlass *holder = NULL;
  Method* recheck_method =  NULL;
  while (k != NULL) {

      //test
      //printf("class name = %s\n",classname->base());
    // lookup through the hierarchy for a method with matching name and sign.
    super_method = InstanceKlass::cast(k)->lookup_method(name, signature);//判断当前方法名与签名在父类中是否吸同名同签名的方法
    if (super_method == NULL) {
      break; // we still have to search for a matching miranda method
    }
    // get the class holding the matching method
    // make sure you use that class for is_override
    InstanceKlass* superk = super_method->method_holder();
    // we want only instance method matches
    // pretend private methods are not in the super vtable
    // since we do override around them: e.g. a.m pub/b.m private/c.m pub,
    // ignore private, c.m pub does override a.m pub
    // For classes that were not javac'd together, we also do transitive overriding around
    // methods that have less accessibility
    if ((!super_method->is_static()) &&
       (!super_method->is_private())) {
      if (superk->is_override(super_method, classloader, classname, THREAD)) {//如果父类方法的访问权限是public或者protected,并且没有static或private修饰，则说明子类重写了父类的方法，此时返回false
        return false;
      // else keep looking for transitive overrides
      }
    }

    // Start with lookup result and continue to search up
    k = superk->super(); // haven't found an override match yet; continue to look
  }

  // if the target method is public or protected it may have a matching
  // miranda method in the super, whose entry it should re-use.
  // Actually, to handle cases that javac would not generate, we need
  // this check for all access permissions.
  InstanceKlass *sk = InstanceKlass::cast(super);
  if (sk->has_miranda_methods()) {
    if (sk->lookup_method_in_all_interfaces(name, signature, Klass::normal) != NULL) {
      return false;  // found a matching miranda; we do not need a new entry
    }
  }
  return true; // found no match; we need a new entry
}

// We can now create the basic Klass* for this klass
    _klass = InstanceKlass::allocate_instance_klass(loader_data,
                                                    vtable_size,
                                                    itable_size,
                                                    info.static_field_size,
                                                    total_oop_map_size2,
                                                    rt,
                                                    access_flags,
                                                    name,
                                                    super_klass(),
                                                    !host_klass.is_null(),
                                                    CHECK_(nullHandle));

\src\share\vm\oops\instanceKlass.cpp
link_class->link_class_impl
//初始化虚表
      // Initialize the vtable and interface table after
      // methods have been rewritten since rewrite may
      // fabricate new Method*s.
      // also does loader constraint checking
      if (!this_oop()->is_shared()) {
        ResourceMark rm(THREAD);
        this_oop->vtable()->initialize_vtable(true, CHECK_false);//初始化虚表
        this_oop->itable()->initialize_itable(true, CHECK_false);
      }

inline InstanceKlass* klassVtable::ik() const {
  Klass* k = _klass();
  assert(k->oop_is_instance(), "not an InstanceKlass");
  return (InstanceKlass*)k;
}
klassVtable(KlassHandle h_klass, void* base, int length) : _klass(h_klass) {
    _tableOffset = (address)base - (address)h_klass(); _length = length;//虚表偏移(InstanceKlass对像大小)
  }

//虚表地址
vtableEntry* table() const      { 
      return (vtableEntry*)(address(_klass()) + _tableOffset); //InstanceKlass对像基址加上InstanceKlass对像大小
  }

int klassVtable::initialize_from_super(KlassHandle super) {
  if (super.is_null()) {
    return 0;
  } else {
    // copy methods from superKlass
    // can't inherit from array class, so must be InstanceKlass
    assert(super->oop_is_instance(), "must be instance klass");
    InstanceKlass* sk = (InstanceKlass*)super();
    klassVtable* superVtable = sk->vtable();
    assert(superVtable->length() <= _length, "vtable too short");
#ifdef ASSERT
    superVtable->verify(tty, true);
#endif
    superVtable->copy_vtable_to(table());
#ifndef PRODUCT
    if (PrintVtables && Verbose) {
      ResourceMark rm;
      tty->print_cr("copy vtable from %s to %s size %d", sk->internal_name(), klass()->internal_name(), _length);
    }
#endif
    return superVtable->length();
  }
}

//
// Revised lookup semantics   introduced 1.3 (Kestrel beta)
void klassVtable::initialize_vtable(bool checkconstraints, TRAPS) {

  // Note:  Arrays can have intermediate array supers.  Use java_super to skip them.
  KlassHandle super (THREAD, klass()->java_super());
  int nofNewEntries = 0;

  if (PrintVtables && !klass()->oop_is_array()) {
    ResourceMark rm(THREAD);
    tty->print_cr("Initializing: %s", _klass->name()->as_C_string());
  }

#ifdef ASSERT
  oop* end_of_obj = (oop*)_klass() + _klass()->size();
  oop* end_of_vtable = (oop*)&table()[_length];
  assert(end_of_vtable <= end_of_obj, "vtable extends beyond end");
#endif

  if (Universe::is_bootstrapping()) {
    // just clear everything
    for (int i = 0; i < _length; i++) table()[i].clear();
    return;
  }

  int super_vtable_len = initialize_from_super(super);
  if (klass()->oop_is_array()) {
    assert(super_vtable_len == _length, "arrays shouldn't introduce new methods");
  } else {
    assert(_klass->oop_is_instance(), "must be InstanceKlass");

    Array<Method*>* methods = ik()->methods();
    int len = methods->length();
    int initialized = super_vtable_len;

    // Check each of this class's methods against super;
    // if override, replace in copy of super vtable, otherwise append to end
    for (int i = 0; i < len; i++) {
      // update_inherited_vtable can stop for gc - ensure using handles
      HandleMark hm(THREAD);
      assert(methods->at(i)->is_method(), "must be a Method*");
      methodHandle mh(THREAD, methods->at(i));

      /*判断是否重写或有虚函数,如果overwrite函数,(方法名字，参数签名 完全一样)，
      也就是替换虚拟表相同顺序的内容*/
      bool needs_new_entry = update_inherited_vtable(ik(), mh, super_vtable_len, -1, checkconstraints, CHECK);

      //needs_new_entry ==true如果符合虚拟函数则顺序添加到虚拟表尾部
      if (needs_new_entry) {
        put_method_at(mh(), initialized);//存放函数
        mh()->set_vtable_index(initialized); // set primary vtable index
        initialized++;
      }
    }
    // update vtable with default_methods
    Array<Method*>* default_methods = ik()->default_methods();
    if (default_methods != NULL) {
      len = default_methods->length();
      if (len > 0) {
        Array<int>* def_vtable_indices = NULL;
        if ((def_vtable_indices = ik()->default_vtable_indices()) == NULL) {
          def_vtable_indices = ik()->create_new_default_vtable_indices(len, CHECK);
        } else {
          assert(def_vtable_indices->length() == len, "reinit vtable len?");
        }
        for (int i = 0; i < len; i++) {
          HandleMark hm(THREAD);
          assert(default_methods->at(i)->is_method(), "must be a Method*");
          methodHandle mh(THREAD, default_methods->at(i));
          
          bool needs_new_entry = update_inherited_vtable(ik(), mh, super_vtable_len, i, checkconstraints, CHECK);

          // needs new entry
          if (needs_new_entry) {
            put_method_at(mh(), initialized);
            def_vtable_indices->at_put(i, initialized); //set vtable index
            initialized++;
          }
        }
      }
    }

    // add miranda methods; it will also return the updated initialized
    // Interfaces do not need interface methods in their vtables
    // This includes miranda methods and during later processing, default methods
    if (!ik()->is_interface()) {
      initialized = fill_in_mirandas(initialized);
    }

    // In class hierarchies where the accessibility is not increasing (i.e., going from private ->
    // package_private -> public/protected), the vtable might actually be smaller than our initial
    // calculation.
    assert(initialized <= _length, "vtable initialization failed");
    for(;initialized < _length; initialized++) {
      put_method_at(NULL, initialized);
    }
    NOT_PRODUCT(verify(tty, true));
  }
}

bool klassVtable::update_inherited_vtable(InstanceKlass* klass, methodHandle target_method,
                                          int super_vtable_len, int default_index,
                                          bool checkconstraints, TRAPS) {
  ResourceMark rm;
  bool allocate_new = true;
  assert(klass->oop_is_instance(), "must be InstanceKlass");

  Array<int>* def_vtable_indices = NULL;
  bool is_default = false;
  // default methods are concrete methods in superinterfaces which are added to the vtable
  // with their real method_holder
  // Since vtable and itable indices share the same storage, don't touch
  // the default method's real vtable/itable index
  // default_vtable_indices stores the vtable value relative to this inheritor
  if (default_index >= 0 ) {
    is_default = true;
    def_vtable_indices = klass->default_vtable_indices();
    assert(def_vtable_indices != NULL, "def vtable alloc?");
    assert(default_index <= def_vtable_indices->length(), "def vtable len?");
  } else {
    assert(klass == target_method()->method_holder(), "caller resp.");
    // Initialize the method's vtable index to "nonvirtual".
    // If we allocate a vtable entry, we will update it to a non-negative number.
    target_method()->set_vtable_index(Method::nonvirtual_vtable_index);
  }

  // Static and <init> methods are never in
  if (target_method()->is_static() || target_method()->name() ==  vmSymbols::object_initializer_name()) {
    return false;
  }

  if (target_method->is_final_method(klass->access_flags())) {
    // a final method never needs a new entry; final methods can be statically
    // resolved and they have to be present in the vtable only if they override
    // a super's method, in which case they re-use its entry
    allocate_new = false;
  } else if (klass->is_interface()) {
    allocate_new = false;  // see note below in needs_new_vtable_entry
    // An interface never allocates new vtable slots, only inherits old ones.
    // This method will either be assigned its own itable index later,
    // or be assigned an inherited vtable index in the loop below.
    // default methods inherited by classes store their vtable indices
    // in the inheritor's default_vtable_indices
    // default methods inherited by interfaces may already have a
    // valid itable index, if so, don't change it
    // overpass methods in an interface will be assigned an itable index later
    // by an inheriting class
    if (!is_default || !target_method()->has_itable_index()) {
      target_method()->set_vtable_index(Method::pending_itable_index);
    }
  }

  // we need a new entry if there is no superclass
  if (klass->super() == NULL) {
    return allocate_new;
  }

  // private methods in classes always have a new entry in the vtable
  // specification interpretation since classic has
  // private methods not overriding
  // JDK8 adds private methods in interfaces which require invokespecial
  if (target_method()->is_private()) {
    return allocate_new;
  }

  // search through the vtable and update overridden entries
  // Since check_signature_loaders acquires SystemDictionary_lock
  // which can block for gc, once we are in this loop, use handles
  // For classfiles built with >= jdk7, we now look for transitive overrides

  Symbol* name = target_method()->name();
  Symbol* signature = target_method()->signature();
  const char* m_method_name = NULL;
  m_method_name = name->as_C_string();
  if (0 == strcmp(m_method_name, "say"))
  {
      printf("target_method name %s\n",m_method_name);
  }
  

  KlassHandle target_klass(THREAD, target_method()->method_holder());
  if (target_klass == NULL) {
    target_klass = _klass;
  }

  Handle target_loader(THREAD, target_klass->class_loader());

  Symbol* target_classname = target_klass->name();

  const char* class_name = target_classname->as_C_string();
  
  //可以在这里判断加载目标类时断点
  if (0 == strcmp(class_name, "Dog") || 0 == strcmp(class_name, "Animal") || 0 == strcmp(class_name, "Cat"))
  {
      printf("update_inherited_vtable %s\n",class_name);

  }
  for(int i = 0; i < super_vtable_len; i++) {
    Method* super_method = method_at(i);
    // Check if method name matches
    m_method_name = super_method->name()->as_C_string();
    printf("super_method name %s\n",m_method_name);

    //判断方法名签名是否与父类中相同
    if (super_method->name() == name && super_method->signature() == signature) {

      // get super_klass for method_holder for the found method
      InstanceKlass* super_klass =  super_method->method_holder();

      //判断是否为重写
      if (is_default
          || ((super_klass->is_override(super_method, target_loader, target_classname, THREAD))
          || ((klass->major_version() >= VTABLE_TRANSITIVE_OVERRIDE_VERSION)
          && ((super_klass = find_transitive_override(super_klass,
                             target_method, i, target_loader,
                             target_classname, THREAD))
                             != (InstanceKlass*)NULL))))
        {
        // Package private methods always need a new entry to root their own
        // overriding. They may also override other methods.
        if (!target_method()->is_package_private()) {
          allocate_new = false;
        }

        if (checkconstraints) {
        // Override vtable entry if passes loader constraint check
        // if loader constraint checking requested
        // No need to visit his super, since he and his super
        // have already made any needed loader constraints.
        // Since loader constraints are transitive, it is enough
        // to link to the first super, and we get all the others.
          Handle super_loader(THREAD, super_klass->class_loader());

          if (target_loader() != super_loader()) {
            ResourceMark rm(THREAD);
            Symbol* failed_type_symbol =
              SystemDictionary::check_signature_loaders(signature, target_loader,
                                                        super_loader, true,
                                                        CHECK_(false));
            if (failed_type_symbol != NULL) {
              const char* msg = "loader constraint violation: when resolving "
                "overridden method \"%s\" the class loader (instance"
                " of %s) of the current class, %s, and its superclass loader "
                "(instance of %s), have different Class objects for the type "
                "%s used in the signature";
              char* sig = target_method()->name_and_sig_as_C_string();
              const char* loader1 = SystemDictionary::loader_name(target_loader());
              char* current = target_klass->name()->as_C_string();
              const char* loader2 = SystemDictionary::loader_name(super_loader());
              char* failed_type_name = failed_type_symbol->as_C_string();
              size_t buflen = strlen(msg) + strlen(sig) + strlen(loader1) +
                strlen(current) + strlen(loader2) + strlen(failed_type_name);
              char* buf = NEW_RESOURCE_ARRAY_IN_THREAD(THREAD, char, buflen);
              jio_snprintf(buf, buflen, msg, sig, loader1, current, loader2,
                           failed_type_name);
              THROW_MSG_(vmSymbols::java_lang_LinkageError(), buf, false);
            }
          }
       }

       put_method_at(target_method(), i);//替换虚函数
       if (!is_default) {
         target_method()->set_vtable_index(i);
       } else {
         if (def_vtable_indices != NULL) {
           def_vtable_indices->at_put(default_index, i);
         }
         assert(super_method->is_default_method() || super_method->is_overpass()
                || super_method->is_abstract(), "default override error");
       }


#ifndef PRODUCT
        if (PrintVtables && Verbose) {
          ResourceMark rm(THREAD);
          char* sig = target_method()->name_and_sig_as_C_string();
          tty->print("overriding with %s::%s index %d, original flags: ",
           target_klass->internal_name(), sig, i);
           super_method->access_flags().print_on(tty);
           if (super_method->is_default_method()) {
             tty->print("default ");
           }
           if (super_method->is_overpass()) {
             tty->print("overpass");
           }
           tty->print("overriders flags: ");
           target_method->access_flags().print_on(tty);
           if (target_method->is_default_method()) {
             tty->print("default ");
           }
           if (target_method->is_overpass()) {
             tty->print("overpass");
           }
           tty->cr();
        }
#endif /*PRODUCT*/
      } else {
        // allocate_new = true; default. We might override one entry,
        // but not override another. Once we override one, not need new
#ifndef PRODUCT
        if (PrintVtables && Verbose) {
          ResourceMark rm(THREAD);
          char* sig = target_method()->name_and_sig_as_C_string();
          tty->print("NOT overriding with %s::%s index %d, original flags: ",
           target_klass->internal_name(), sig,i);
           super_method->access_flags().print_on(tty);
           if (super_method->is_default_method()) {
             tty->print("default ");
           }
           if (super_method->is_overpass()) {
             tty->print("overpass");
           }
           tty->print("overriders flags: ");
           target_method->access_flags().print_on(tty);
           if (target_method->is_default_method()) {
             tty->print("default ");
           }
           if (target_method->is_overpass()) {
             tty->print("overpass");
           }
           tty->cr();
        }
#endif /*PRODUCT*/
      }
    }
  }
  return allocate_new;//如果没有与父类中相同的函数并且满足虚函数特性就返回true
}

void klassVtable::put_method_at(Method* m, int index) {
#ifndef PRODUCT
  if (PrintVtables && Verbose) {
    ResourceMark rm;
    const char* sig = (m != NULL) ? m->name_and_sig_as_C_string() : "<NULL>";
    tty->print("adding %s at index %d, flags: ", sig, index);
    if (m != NULL) {
      m->access_flags().print_on(tty);
      if (m->is_default_method()) {
        tty->print("default ");
      }
      if (m->is_overpass()) {
        tty->print("overpass");
      }
    }
    tty->cr();
  }
#endif
  table()[index].set(m);// 将函数地址放入虚表
}

Animal  父类
say 0x14890250 index 5
table()    0x14890520  vtableEntry *
table()[index]    {_method=0x14890250 }    vtableEntry

Dog
say 0x148906e0 index 5
table()    0x14890920 vtableEntry *
table()[index]    {_method=0x14890250 }    vtableEntry //没有替换前与Animal中say函数地址相同
table()[index]    {_method=0x148906e0 }    vtableEntry //替换后为Dog的say函数地址