hotspot中的Java类对象如何保存虚函数

定义

在Java中，几乎所有可被继承的函数被称为虚函数。

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In HotSpot, a virtual method is essentially:

Any non-static, non-private, non-final instance method whose invocation target is determined at runtime based on the actual class of the object.

还是以Dog类为例

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public class Dog implements Animal {
	...
	public String speak(){
        return "Woof!";
    }
    ...
}

这里的speak()函数就是Java的虚函数，编译后的字节码文件也使用invokevirual字节码来执行该函数

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27: invokevirtual #53                 // Method speak:()Ljava/lang/String;

在《Java虚拟机规范》上对invokevirtual的解释如下

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Invoke instance method; dispatch based on class

Klass中虚函数字段

在hotspot中使用Klass的虚函数使用vtable()来描述

C++ 复制代码

class Klass : public Metadata {
...
protected:
	// vtable length
  	int _vtable_len;
	// vtables
  	klassVtable vtable() const;//返回KlassVtable对象
  	...
}

在hotspot解析字节码的函数部分中会筛选出虚函数部分

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#0  klassVtable::compute_vtable_size_and_num_mirandas (
    
#1  0x00007ffff61b1927 in ClassFileParser::post_process_parsed_stream (

#2  0x00007ffff61afcf0 in ClassFileParser::ClassFileParser (

在void klassVtable::compute_vtable_size_and_num_mirandas函数中根据函数的访问权限找出虚函数

C++ 复制代码

void klassVtable::compute_vtable_size_and_num_mirandas(
	...
	if (needs_new_vtable_entry(method, super, classloader, classname, class_flags, major_version)) {//判断的昂前函数是否为虚函数
      ...
      vtable_length += vtableEntry::size(); // we need a new entry
    }
    ...
 }

我们现在针对java/lang/Object进行gdb调试，我们来看看needs_new_vtable_entry函数中可以引发vtable的method

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Thread 2 "java" hit Breakpoint 4, klassVtable::compute_vtable_size_and_num_mirandas (vtable_length_ret=0x7ffff59fdf28, num_new_mirandas=0x7ffff59fdf30, all_mirandas=0x7ffff0034af0, super=0x0, methods=0x7fffe8800460, class_flags=..., major_version=61, classloader=..., classname=0x7ffff40210f0, local_interfaces=0x7fffe8800058) at /home/jx/src/openjdk/src/hotspot/share/oops/klassVtable.cpp:87
87            vtable_length += vtableEntry::size(); // we need a new entry
(gdb) print method->
Display all 200 possibilities? (y or n)
(gdb) print method->name()->as_C_string()
$7 = 0x7ffff0034bd0 "finalize"
(gdb) display method->name()->as_C_string()
1: method->name()->as_C_string() = 0x7ffff0034be0 "finalize"
(gdb) c
Continuing.
...
1: method->name()->as_C_string() = 0x7ffff0034bf0 "equals"
(gdb) c
Continuing.
...
1: method->name()->as_C_string() = 0x7ffff0034c00 "toString"
(gdb) c
Continuing.

...
1: method->name()->as_C_string() = 0x7ffff0034c10 "hashCode"
(gdb) c
Continuing.

...
1: method->name()->as_C_string() = 0x7ffff0034c20 "clone"
(gdb)

可以看到Object.java中的5个函数属于虚函数

java 复制代码

@IntrinsicCandidate
public native int hashCode();

public boolean equals(Object obj) {
	return (this == obj);
}

@IntrinsicCandidate
protected native Object clone() throws CloneNotSupportedException;

public String toString() {
	return getClass().getName() + "@" + Integer.toHexString(hashCode());
}

@Deprecated(since="9")
protected void finalize() throws Throwable { }

java/lang/Object除此之外的函数声明如下

java 复制代码

@IntrinsicCandidate
public final native Class<?> getClass();

@IntrinsicCandidate
public final native void notify();

@IntrinsicCandidate
public final native void notifyAll();

public final void wait() throws InterruptedException {

public final void wait(long timeoutMillis, int nanos) throws InterruptedException {

public final native void wait(long timeoutMillis) throws InterruptedException;

所有函数都有final声明，表明这些函数不能被继承，即不属于虚函数

虚函数字段的内存布局

C++ 复制代码

class klassVtable {
  Klass*       _klass;            // my klass
  int          _tableOffset;      // offset of start of vtable data within klass
  int          _length;
  public:
      klassVtable(Klass* klass, void* base, int length) 
          : _klass(klass) {
        		_tableOffset = (address)base - (address)klass; 
              	_length = length;
      }

可以通过Klass中的vtable()函数获得内存布局

C++ 复制代码

inline klassVtable Klass::vtable() const {
  return klassVtable(const_cast<Klass*>(this), start_of_vtable(), vtable_length() / vtableEntry::size());
}

/*start_of_vtable()实现如下*/
inline vtableEntry* Klass::start_of_vtable() const {
  return (vtableEntry*) ((address)this + in_bytes(vtable_start_offset()));
}

/*vtable_start_offset()实现如下*/
inline ByteSize Klass::vtable_start_offset() {
  return in_ByteSize(InstanceKlass::header_size() * wordSize);
}

内存布局如下

虚函数位置的摆放算法如下

C++ 复制代码

void klassVtable::put_method_at(Method* m, int index) {
	...
	table()[index].set(m);
}

vtableEntry* table() const      { return (vtableEntry*)(address(_klass) + _tableOffset); }

klassVtable(Klass* klass, void* base, int length) : _klass(klass) {
    _tableOffset = (address)base - (address)klass; _length = length;
}

可以看到table()的地址是start_of_vtable()，也就是说从table()开始摆放虚函数地址

C++ 复制代码

class vtableEntry {
	...
	private:
  		Method* _method;
  		void set(Method* method)  { assert(method != NULL, "use clear"); _method = method; }
  	...
}

由于vtableEntry只有一个成员变量_method，因此可以将其直接作为插槽插入到虚函数表中，如下图