java接口方法调用源码分析

对于这样的java代码

pubic interface TestInterface {
  int foo();
  int foo1();
}

pubic interface TestClass implement TestInterface{
  public int foo(){
    int a  = 0;
    reurn a;
  }

   public int foo1(){
    int a  = 1;
    reurn a;
  }
}

TestInterface i = new TestClass();

i.foo();

javac之后会生成invoke_interface 的字节码.相关的实现位于templateTable::invokeInterface(int)生成的代码中.


java对象的的内存布局.

首先,变量i指向的内存布局是这样的.

[oopDesc][长度与类相关的内存区域]


oopDesc 包含2个成员, markWord和 klass.其中klass在未启用指针压缩时,指向该对象的类信息.
这个"类信息"不是java.lang.Class,而是虚拟机使用的内部数据.


klass指向的内存区域布局,参见(instanceKlass.hpp)

[klassOopDesc][instanceKlass][javaVtable][javaItable][静态成员][oop map]


klassOopDesc继承于oopDesc,添加了方法,但是没有新成员变量,所以内存结构一样.instanceKlass包含大量的类信息.其中,这里需要用到的就有 vtable_length, 其值就是javaVtable的长度.



[javaVtable] ==> vtable_length 个 method指针.
对于这里的例子,应该是以这个顺序

(Object类的方法)================
finalize
equals
toString
hashCode
clone
(TestClass类的方法)==
foo
foo1




java Itable内存布局,首先是该类说实现的所有接口类,以及该接口类的方法的偏移位置.最后以0表示接口类信息结束, 然后尾随的就是所有接口类的每个方法

interface0, offset
interface1, offset
.
.
.
interfacen, offset
0        0
========================
interface0_方法0
interface0_方法1
.
.
.
interface1_方法0
interface1_方法1
.
.
.


在上面的额示例代码里.itable就是


指向TestInterface指针, 偏移量---
0,                     0                |
指向foo方法的指针<--------------
指向foo1方法的指针



来看看invokeinterface的源码,

首先是和其他方法调用一样.
prepare_invoke()先去缓存找方法, 找不到就解析.连接


重点在这里
lookup_interface_method

movl(scan_temp, Address(recv_klass, instanceKlass::vtable_length_offset() * wordSize));

//scan_temp = (instanceKlass*)(recv_klass+ sizeof(klassOopDesc) )->_vtable_len

lea(scan_temp, Address(recv_klass, scan_temp, times_vte_scale, vtable_base));

//scan_temp =  recv_klass + vtable_base + scan_temp * sizeof(vtableEntry); // end of vtable, vtable的后面就是itable了


lea(recv_klass, Address(recv_klass, itable_index, Address::times_ptr, itentry_off));

//recv_klass = recv_klass + itable_index * sizeof(itableMethodEntry) + offset_of(itableMethodEntry, _method)  @1

for (int peel = 1; peel >= 0; peel--) {
    movptr(method_result, Address(scan_temp, itableOffsetEntry::interface_offset_in_bytes()));
    cmpptr(intf_klass, method_result);

    if (peel) {
      jccb(Assembler::equal, found_method);
    } else {
      jccb(Assembler::notEqual, search);
      // (invert the test to fall through to found_method...)
    }

    if (!peel)  break;

    bind(search);

    // Check that the previous entry is non-null.  A null entry means that
    // the receiver class doesn't implement the interface, and wasn't the
    // same as when the caller was compiled.
    testptr(method_result, method_result);
    jcc(Assembler::zero, L_no_such_interface);
    addptr(scan_temp, scan_step);

	
  }

  bind(found_method);
//上面的代码简直坑爹展开一下


	movptr(method_result, Address(scan_temp, itableOffsetEntry::interface_offset_in_bytes()));
//method_result = scan_temp->_interface
    cmpptr(intf_klass, method_result);
	jccb(Assembler::equal, found_method);

//if(intf_klass == method_result)
 // goto found_method;

	bind(search);
	testptr(method_result, method_result);
    jcc(Assembler::zero, L_no_such_interface);
//if(method_result == 0)
//   goto L_no_such_interface;

    addptr(scan_temp, scan_step);
// scan_temp += sizeof(itableOffsetEntry);
	movptr(method_result, Address(scan_temp, itableOffsetEntry::interface_offset_in_bytes()));
//method_result = scan_temp->_interface
    cmpptr(intf_klass, method_result);
    jccb(Assembler::notEqual, search);
    //if(intf_klass != result)
//       goto search;



	 bind(found_method);

  // Got a hit.
  movl(scan_temp, Address(scan_temp, itableOffsetEntry::offset_offset_in_bytes()));
  //scan_temp = ((itableOffsetEntry*)scan_temp)->_offset
  movptr(method_result, Address(recv_klass, scan_temp, Address::times_1));
  //method_result= recv_klass + scan_temp->_offset @2

//@1+@2 ==> 
//method_result = ((itableMethodEntry*)(recv_klass + scan_temp->_offset))[itable_index]._method

 for (scan = klass->itable(); scan->interface() != NULL; scan += sizeof(itableOffsetEntry)) {
     if (scan->interface() == intf) {
       result = (klass + scan->offset() + itable_index);
     }
   }

最后顺便提一句.vtable里是有接口方法的的.

所以类似

引用

TestClass t = new TestClass();//或者是TestClass的子类
t.foo();

这种事走vtable的,具体的javap看一下,他生成的是invokeVirtual