以Unsafe类的下列方法为例子
public native boolean getBoolean(Object var1, long var2);
java本地方法以jni调用实现(jni如何调用先不做介绍),按照jni中java方法和c方法的对应关系,那么在c中名称应该为
jboolean, Unsafe_GetBoolean(JNIEnv *env, jobject unsafe, jobject obj, jlong offset)
实际为
jboolean, Unsafe_Get##Boolean(JNIEnv *env, jobject unsafe, jobject obj, jlong offset)
定义在宏中,使用了##连接操作符,内容为
UNSAFE_ENTRY(jboolean, Unsafe_Get##Boolean(JNIEnv *env, jobject unsafe, jobject obj, jlong offset)) \
UnsafeWrapper("Unsafe_Get"#Boolean); \
GET_FIELD(obj, offset, jboolean, v); \
return v; \
UNSAFE_END \
原则是调用
GET_FIELD(obj, offset, jboolean, v)
查看其源码
#define GET_FIELD(obj, offset, type_name, v) \
oop p = JNIHandles::resolve(obj); \
type_name v = *(type_name*)index_oop_from_field_offset_long(p, offset)
再看看index_oop_from_field_offset_long方法
inline void* index_oop_from_field_offset_long(oop p, jlong field_offset) {
jlong byte_offset = field_offset_to_byte_offset(field_offset);
#ifdef ASSERT
if (p != NULL) {
assert(byte_offset >= 0 && byte_offset <= (jlong)MAX_OBJECT_SIZE, "sane offset");
if (byte_offset == (jint)byte_offset) {
void* ptr_plus_disp = (address)p + byte_offset;
assert((void*)p->obj_field_addr((jint)byte_offset) == ptr_plus_disp,
"raw [ptr+disp] must be consistent with oop::field_base");
}
jlong p_size = HeapWordSize * (jlong)(p->size());
assert(byte_offset < p_size, err_msg("Unsafe access: offset " INT64_FORMAT " > object's size " INT64_FORMAT, byte_offset, p_size));
}
#endif
if (sizeof(char*) == sizeof(jint)) // (this constant folds!)
return (address)p + (jint) byte_offset;
else
return (address)p + byte_offset;
}
就是传入对象的地址,并告诉返回类型的偏移量,然后c++通过指针及偏移返回对应的地址初始位置,再根据类型(确定指针类型)强转成对应类型的值返回,地址+偏移=对应数据的地址
下面确认下java传入的fieldOffset是啥值
package com.jinrong.zaxiang.refect;
import java.lang.reflect.Field;
/**
* @author wocan23
* @create 2019/9/24 下午5:56
*/
public class RefectTest {
public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
User user = new User();
user.setAge(2);
user.setName("zss");
user.setFlagStudent(true);
Class aClass = user.getClass();
Field flagStudentField = aClass.getDeclaredField("flagStudent");
flagStudentField.setAccessible(true);
boolean aBoolean = flagStudentField.getBoolean(user);
Field nameField = aClass.getDeclaredField("name");
nameField.setAccessible(true);
Object o = nameField.get(user);
Field ageField = aClass.getDeclaredField("age");
ageField.setAccessible(true);
Object o1 = ageField.getInt(user);
}
}
class User{
private int age; // offset 12
private String name; // offset 20
private boolean flagStudent; // offset 16
public boolean isFlagStudent() {
return flagStudent;
}
public void setFlagStudent(boolean flagStudent) {
this.flagStudent = flagStudent;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
跟断点发现offset为16,再看下name和age得offset为多少20和12 调换下顺序,偏移量仍旧未变
再增加一个变量
class User{
private String name; // offset 20
private int age; // offset 12
private boolean flagStudent; // offset 16
private boolean flagMale; // offset 17
发现只是新增得变了,其实际字节码中顺序和声明顺序不一样
另外成员是从12开始,64位Java虚拟机引入了压缩指针概念(对应虚拟机选项 -XX:+UseCompressedOops,默认开启),将堆中原本64位的Java对象指针压缩成32位的。
这样一来,对象头的类型指针也会被压缩成32位,使得对象头大小从16字节降低为12字节