深入分析Java反射(七)-简述反射调用的底层实现
前提
Java反射的API在JavaSE1.7的时候已经基本完善，但是本文编写的时候使用的是Oracle JDK11，因为JDK11对于sun包下的源码也上传了，可以直接通过IDE查看对应的源码和进行Debug。
本文主要介绍反射调用的底层实现，当然还没有能力分析JVM的实现，这里只分析到最终Native方法的调用点。底层会依赖到Unsafe类，可以的话可以看下笔者之前写的一篇文章《神奇的魔法类和双刃剑-Unsafe》。
反射调用的底层实现探究
主要考虑下面的情况：
- 属性操作：
java.lang.reflect.Field#set(Object obj, Object value)和java.lang.reflect.Field#get(Object obj)。 - 构造器调用：
java.lang.reflect.Constructor#newInstance(Object ... initargs)。 - 方法调用：
java.lang.reflect.Method#invoke(Object obj, Object... args)。
处理属性操作的底层实现
属性操作方法Field#set(Object obj, Object value)和Field#get(Object obj)底层都是委托到jdk.internal.reflect.FieldAccessor实现：
public interface FieldAccessor {
/** Matches specification in {@link java.lang.reflect.Field} */
public Object get(Object obj) throws IllegalArgumentException;
/** Matches specification in {@link java.lang.reflect.Field} */
public boolean getBoolean(Object obj) throws IllegalArgumentException;
/** Matches specification in {@link java.lang.reflect.Field} */
public byte getByte(Object obj) throws IllegalArgumentException;
/** Matches specification in {@link java.lang.reflect.Field} */
public char getChar(Object obj) throws IllegalArgumentException;
/** Matches specification in {@link java.lang.reflect.Field} */
public short getShort(Object obj) throws IllegalArgumentException;
/** Matches specification in {@link java.lang.reflect.Field} */
public int getInt(Object obj) throws IllegalArgumentException;
/** Matches specification in {@link java.lang.reflect.Field} */
public long getLong(Object obj) throws IllegalArgumentException;
/** Matches specification in {@link java.lang.reflect.Field} */
public float getFloat(Object obj) throws IllegalArgumentException;
/** Matches specification in {@link java.lang.reflect.Field} */
public double getDouble(Object obj) throws IllegalArgumentException;
/** Matches specification in {@link java.lang.reflect.Field} */
public void set(Object obj, Object value)
throws IllegalArgumentException, IllegalAccessException;
/** Matches specification in {@link java.lang.reflect.Field} */
public void setBoolean(Object obj, boolean z)
throws IllegalArgumentException, IllegalAccessException;
/** Matches specification in {@link java.lang.reflect.Field} */
public void setByte(Object obj, byte b)
throws IllegalArgumentException, IllegalAccessException;
/** Matches specification in {@link java.lang.reflect.Field} */
public void setChar(Object obj, char c)
throws IllegalArgumentException, IllegalAccessException;
/** Matches specification in {@link java.lang.reflect.Field} */
public void setShort(Object obj, short s)
throws IllegalArgumentException, IllegalAccessException;
/** Matches specification in {@link java.lang.reflect.Field} */
public void setInt(Object obj, int i)
throws IllegalArgumentException, IllegalAccessException;
/** Matches specification in {@link java.lang.reflect.Field} */
public void setLong(Object obj, long l)
throws IllegalArgumentException, IllegalAccessException;
/** Matches specification in {@link java.lang.reflect.Field} */
public void setFloat(Object obj, float f)
throws IllegalArgumentException, IllegalAccessException;
/** Matches specification in {@link java.lang.reflect.Field} */
public void setDouble(Object obj, double d)
throws IllegalArgumentException, IllegalAccessException;
}
FieldAccessor接口有很多的实现，FieldAccessor接口实例是通过jdk.internal.reflect.ReflectionFactory这个工厂构造的：
public FieldAccessor newFieldAccessor(Field field, boolean override) {
checkInitted();
Field root = langReflectAccess.getRoot(field);
if (root != null) {
// FieldAccessor will use the root unless the modifiers have
// been overrridden
if (root.getModifiers() == field.getModifiers() || !override) {
field = root;
}
}
return UnsafeFieldAccessorFactory.newFieldAccessor(field, override);
}
最终委托到UnsafeFieldAccessorFactory#newFieldAccessor()：
class UnsafeFieldAccessorFactory {
static FieldAccessor newFieldAccessor(Field field, boolean override) {
Class<?> type = field.getType();
boolean isStatic = Modifier.isStatic(field.getModifiers());
boolean isFinal = Modifier.isFinal(field.getModifiers());
boolean isVolatile = Modifier.isVolatile(field.getModifiers());
boolean isQualified = isFinal || isVolatile;
boolean isReadOnly = isFinal && (isStatic || !override);
if (isStatic) {
// This code path does not guarantee that the field's
// declaring class has been initialized, but it must be
// before performing reflective operations.
UnsafeFieldAccessorImpl.unsafe.ensureClassInitialized(field.getDeclaringClass());
if (!isQualified) {
if (type == Boolean.TYPE) {
return new UnsafeStaticBooleanFieldAccessorImpl(field);
} else if (type == Byte.TYPE) {
return new UnsafeStaticByteFieldAccessorImpl(field);
} else if (type == Short.TYPE) {
return new UnsafeStaticShortFieldAccessorImpl(field);
} else if (type == Character.TYPE) {
return new UnsafeStaticCharacterFieldAccessorImpl(field);
} else if (type == Integer.TYPE) {
return new UnsafeStaticIntegerFieldAccessorImpl(field);
} else if (type == Long.TYPE) {
return new UnsafeStaticLongFieldAccessorImpl(field);
} else if (type == Float.TYPE) {
return new UnsafeStaticFloatFieldAccessorImpl(field);
} else if (type == Double.TYPE) {
return new UnsafeStaticDoubleFieldAccessorImpl(field);
} else {
return new UnsafeStaticObjectFieldAccessorImpl(field);
}
} else {
if (type == Boolean.TYPE) {
return new UnsafeQualifiedStaticBooleanFieldAccessorImpl(field, isReadOnly);
} else if (type == Byte.TYPE) {
return new UnsafeQualifiedStaticByteFieldAccessorImpl(field, isReadOnly);
} else if (type == Short.TYPE) {
return new UnsafeQualifiedStaticShortFieldAccessorImpl(field, isReadOnly);
} else if (type == Character.TYPE) {
return new UnsafeQualifiedStaticCharacterFieldAccessorImpl(field, isReadOnly);
} else if (type == Integer.TYPE) {
return new UnsafeQualifiedStaticIntegerFieldAccessorImpl(field, isReadOnly);
} else if (type == Long.TYPE) {
return new UnsafeQualifiedStaticLongFieldAccessorImpl(field, isReadOnly);
} else if (type == Float.TYPE) {
return new UnsafeQualifiedStaticFloatFieldAccessorImpl(field, isReadOnly);
} else if (type == Double.TYPE) {
return new UnsafeQualifiedStaticDoubleFieldAccessorImpl(field, isReadOnly);
} else {
return new UnsafeQualifiedStaticObjectFieldAccessorImpl(field, isReadOnly);
}
}
} else {
if (!isQualified) {
if (type == Boolean.TYPE) {
return new UnsafeBooleanFieldAccessorImpl(field);
} else if (type == Byte.TYPE) {
return new UnsafeByteFieldAccessorImpl(field);
} else if (type == Short.TYPE) {
return new UnsafeShortFieldAccessorImpl(field);
} else if (type == Character.TYPE) {
return new UnsafeCharacterFieldAccessorImpl(field);
} else if (type == Integer.TYPE) {
return new UnsafeIntegerFieldAccessorImpl(field);
} else if (type == Long.TYPE) {
return new UnsafeLongFieldAccessorImpl(field);
} else if (type == Float.TYPE) {
return new UnsafeFloatFieldAccessorImpl(field);
} else if (type == Double.TYPE) {
return new UnsafeDoubleFieldAccessorImpl(field);
} else {
return new UnsafeObjectFieldAccessorImpl(field);
}
} else {
if (type == Boolean.TYPE) {
return new UnsafeQualifiedBooleanFieldAccessorImpl(field, isReadOnly);
} else if (type == Byte.TYPE) {
return new UnsafeQualifiedByteFieldAccessorImpl(field, isReadOnly);
} else if (type == Short.TYPE) {
return new UnsafeQualifiedShortFieldAccessorImpl(field, isReadOnly);
} else if (type == Character.TYPE) {
return new UnsafeQualifiedCharacterFieldAccessorImpl(field, isReadOnly);
} else if (type == Integer.TYPE) {
return new UnsafeQualifiedIntegerFieldAccessorImpl(field, isReadOnly);
} else if (type == Long.TYPE) {
return new UnsafeQualifiedLongFieldAccessorImpl(field, isReadOnly);
} else if (type == Float.TYPE) {
return new UnsafeQualifiedFloatFieldAccessorImpl(field, isReadOnly);
} else if (type == Double.TYPE) {
return new UnsafeQualifiedDoubleFieldAccessorImpl(field, isReadOnly);
} else {
return new UnsafeQualifiedObjectFieldAccessorImpl(field, isReadOnly);
}
}
}
}
}
这里注意一下属性修饰符的判断：
- isStatic：静态属性，也就是static关键字修饰的属性。
- isFinal：final关键字修饰的属性。
- isVolatile：valatile关键字修饰的属性。
- isQualified：valatile关键字或者final关键字修饰的属性。
- isReadOnly：是否只读属性，final关键字修饰的属性或者static关键字修饰并且不能覆盖(override = false)的属性。
通过上面修饰符做判断，得到最终的FieldAccessor实现。这里挑一个例子进行分析，例如一个普通非静态没有volatile和final关键字修饰属性最终就会得到UnsafeObjectFieldAccessorImpl的实例：
class UnsafeObjectFieldAccessorImpl extends UnsafeFieldAccessorImpl {
UnsafeObjectFieldAccessorImpl(Field field) {
super(field);
}
public Object get(Object obj) throws IllegalArgumentException {
ensureObj(obj);
return unsafe.getObject(obj, fieldOffset);
}
public void set(Object obj, Object value)
throws IllegalArgumentException, IllegalAccessException{
ensureObj(obj);
if (isFinal) {
throwFinalFieldIllegalAccessException(value);
}
if (value != null) {
if (!field.getType().isAssignableFrom(value.getClass())) {
throwSetIllegalArgumentException(value);
}
}
unsafe.putObject(obj, fieldOffset, value);
}
public boolean getBoolean(Object obj) throws IllegalArgumentException {
throw newGetBooleanIllegalArgumentException();
}
public byte getByte(Object obj) throws IllegalArgumentException {
throw newGetByteIllegalArgumentException();
}
// 省略其他直接抛出异常的方法
}
可见UnsafeObjectFieldAccessorImpl中除了get(Object obj)和set(Object obj, Object value)方法，其他方法都是直接抛出IllegalArgumentException。而get(Object obj)和set(Object obj, Object value)底层分别依赖于jdk.internal.misc.Unsafe的putObject(obj, fieldOffset, value)和getObject(obj, fieldOffset)方法。而属性的内存偏移地址是在UnsafeObjectFieldAccessorImpl的父类UnsafeFieldAccessorImpl的构造函数中计算出来的：
abstract class UnsafeFieldAccessorImpl extends FieldAccessorImpl {
static final Unsafe unsafe = Unsafe.getUnsafe();
protected final Field field;
protected final long fieldOffset;
protected final boolean isFinal;
UnsafeFieldAccessorImpl(Field field) {
this.field = field;
if (Modifier.isStatic(field.getModifiers()))
fieldOffset = unsafe.staticFieldOffset(field);
else
fieldOffset = unsafe.objectFieldOffset(field);
isFinal = Modifier.isFinal(field.getModifiers());
}
// 省略其他方法
}
这里可以做个小结，属性反射操作Field的setXX和getXX方法最终委托到jdk.internal.misc.Unsafe的putXX和getXX方法，而属性的内存偏移地址是通过jdk.internal.misc.Unsafe的staticFieldBase()、staticFieldOffset和objectFieldOffset几个方法计算的。
处理构造器调用的底层实现
Constructor#newInstance()方法调用依赖到ConstructorAccessor：
public T newInstance(Object ... initargs)
throws InstantiationException, IllegalAccessException,
IllegalArgumentException, InvocationTargetException
{
if (!override) {
Class<?> caller = Reflection.getCallerClass();
checkAccess(caller, clazz, clazz, modifiers);
}
if ((clazz.getModifiers() & Modifier.ENUM) != 0)
throw new IllegalArgumentException("Cannot reflectively create enum objects");
ConstructorAccessor ca = constructorAccessor; // read volatile
if (ca == null) {
ca = acquireConstructorAccessor();
}
@SuppressWarnings("unchecked")
T inst = (T) ca.newInstance(initargs);
return inst;
}
// ConstructorAccessor接口
public interface ConstructorAccessor {
/** Matches specification in {@link java.lang.reflect.Constructor} */
public Object newInstance(Object[] args)
throws InstantiationException,
IllegalArgumentException,
InvocationTargetException;
}
而获取ConstructorAccessor实例也是通过反射工厂类ReflectionFactory，具体是ReflectionFactory#newConstructorAccessor：
public ConstructorAccessor newConstructorAccessor(Constructor<?> c) {
checkInitted();
Class<?> declaringClass = c.getDeclaringClass();
// 抽象方法会进入此if分支
if (Modifier.isAbstract(declaringClass.getModifiers())) {
return new InstantiationExceptionConstructorAccessorImpl(null);
}
// 宿主类直接是Class类型，则无法实例化
if (declaringClass == Class.class) {
return new InstantiationExceptionConstructorAccessorImpl
("Can not instantiate java.lang.Class");
}
// use the root Constructor that will not cache caller class
Constructor<?> root = langReflectAccess.getRoot(c);
if (root != null) {
c = root;
}
// 当前声明构造的宿主类是ConstructorAccessorImpl的子类
if (Reflection.isSubclassOf(declaringClass,
ConstructorAccessorImpl.class)) {
return new BootstrapConstructorAccessorImpl(c);
}
//
if (noInflation && !ReflectUtil.isVMAnonymousClass(c.getDeclaringClass())) {
return new MethodAccessorGenerator().
generateConstructor(c.getDeclaringClass(),
c.getParameterTypes(),
c.getExceptionTypes(),
c.getModifiers());
} else {
NativeConstructorAccessorImpl acc =
new NativeConstructorAccessorImpl(c);
DelegatingConstructorAccessorImpl res =
new DelegatingConstructorAccessorImpl(acc);
acc.setParent(res);
return res;
}
}
可见最终得到的ConstructorAccessor实例为DelegatingConstructorAccessorImpl，而DelegatingConstructorAccessorImpl只是一个委托实现，底层是调用NativeConstructorAccessorImpl：
class NativeConstructorAccessorImpl extends ConstructorAccessorImpl {
private final Constructor<?> c;
private DelegatingConstructorAccessorImpl parent;
private int numInvocations;
NativeConstructorAccessorImpl(Constructor<?> c) {
this.c = c;
}
public Object newInstance(Object[] args)
throws InstantiationException,
IllegalArgumentException,
InvocationTargetException
{
// We can't inflate a constructor belonging to a vm-anonymous class
// because that kind of class can't be referred to by name, hence can't
// be found from the generated bytecode.
if (++numInvocations > ReflectionFactory.inflationThreshold()
&& !ReflectUtil.isVMAnonymousClass(c.getDeclaringClass())) {
ConstructorAccessorImpl acc = (ConstructorAccessorImpl)
new MethodAccessorGenerator().
generateConstructor(c.getDeclaringClass(),
c.getParameterTypes(),
c.getExceptionTypes(),
c.getModifiers());
parent.setDelegate(acc);
}
return newInstance0(c, args);
}
void setParent(DelegatingConstructorAccessorImpl parent) {
this.parent = parent;
}
// 这个就是最终构造实例化对象的native方法
private static native Object newInstance0(Constructor<?> c, Object[] args)
throws InstantiationException,
IllegalArgumentException,
InvocationTargetException;
}
NativeConstructorAccessorImpl#newInstance0()就是最终构造实例化对象的Native方法。当然有例外的情况，例如非正常调用下，如果构造器的宿主类是一个抽象类，那么最终会返回一个InstantiationExceptionConstructorAccessorImpl实例，里面直接抛出InstantiationException异常。
处理方法调用的底层实现
Method#invoke()调用依赖于MethodAccessor：
// MethodAccessor接口
public interface MethodAccessor {
/** Matches specification in {@link java.lang.reflect.Method} */
public Object invoke(Object obj, Object[] args)
throws IllegalArgumentException, InvocationTargetException;
}
public Object invoke(Object obj, Object... args)
throws IllegalAccessException, IllegalArgumentException,
InvocationTargetException{
if (!override) {
Class<?> caller = Reflection.getCallerClass();
checkAccess(caller, clazz,
Modifier.isStatic(modifiers) ? null : obj.getClass(),
modifiers);
}
MethodAccessor ma = methodAccessor; // read volatile
if (ma == null) {
ma = acquireMethodAccessor();
}
return ma.invoke(obj, args);
}
获取MethodAccessor实例的逻辑和前两节类似，是通过ReflectionFactory#newMethodAccessor()：
public MethodAccessor newMethodAccessor(Method method) {
checkInitted();
if (Reflection.isCallerSensitive(method)) {
Method altMethod = findMethodForReflection(method);
if (altMethod != null) {
method = altMethod;
}
}
// use the root Method that will not cache caller class
Method root = langReflectAccess.getRoot(method);
if (root != null) {
method = root;
}
if (noInflation && !ReflectUtil.isVMAnonymousClass(method.getDeclaringClass())) {
return new MethodAccessorGenerator().
generateMethod(method.getDeclaringClass(),
method.getName(),
method.getParameterTypes(),
method.getReturnType(),
method.getExceptionTypes(),
method.getModifiers());
} else {
NativeMethodAccessorImpl acc =
new NativeMethodAccessorImpl(method);
DelegatingMethodAccessorImpl res =
new DelegatingMethodAccessorImpl(acc);
acc.setParent(res);
return res;
}
}
最终会委托到NativeMethodAccessorImpl#invoke(Object obj, Object[] args)：
class NativeMethodAccessorImpl extends MethodAccessorImpl {
private final Method method;
private DelegatingMethodAccessorImpl parent;
private int numInvocations;
NativeMethodAccessorImpl(Method method) {
this.method = method;
}
public Object invoke(Object obj, Object[] args)
throws IllegalArgumentException, InvocationTargetException
{
// We can't inflate methods belonging to vm-anonymous classes because
// that kind of class can't be referred to by name, hence can't be
// found from the generated bytecode.
if (++numInvocations > ReflectionFactory.inflationThreshold()
&& !ReflectUtil.isVMAnonymousClass(method.getDeclaringClass())) {
MethodAccessorImpl acc = (MethodAccessorImpl)
new MethodAccessorGenerator().
generateMethod(method.getDeclaringClass(),
method.getName(),
method.getParameterTypes(),
method.getReturnType(),
method.getExceptionTypes(),
method.getModifiers());
parent.setDelegate(acc);
}
return invoke0(method, obj, args);
}
void setParent(DelegatingMethodAccessorImpl parent) {
this.parent = parent;
}
private static native Object invoke0(Method m, Object obj, Object[] args);
}
而NativeMethodAccessorImpl#invoke0()就是方法调用的最终调用的Native方法。
小结
学习知识过程总是阶梯式上升的，JDK中的类库设计也类似这样，如果提前熟悉Unsafe类的相关方法，其实反射调用的底层实现也能够相对轻易地理解。属性、构造和方法反射调用底层的实现(只考虑正常调用的情况下)如下：
- 对于属性(Field)：
Field#setXX()和Field#getXX()分别对应Unsafe的putXX()和getXX()方法，也就是说完全依赖Unsafe中的Native方法。 - 对于构造(Constructor)：
Constructor#newInstance()底层调用NativeConstructorAccessorImpl#newInstance0()。 - 对于方法(Method)：
Method#invoke()底层调用NativeMethodAccessorImpl#invoke0()
个人博客
(本文完 e-a-20181216 c-1-d)
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