反射获取类实例
首先调用了 java.lang.Class 的静态方法，获取类信息。
@CallerSensitive
public static Class<?> forName(String className) throws ClassNotFoundException {
// 先通过反射，获取调用进来的类信息，从而获取当前的 classLoader
Class<?> caller = Reflection.getCallerClass();
// 调用native方法进行获取class信息
return forName0(className, true, ClassLoader.getClassLoader(caller), caller);
}
forName()反射获取类信息，并没有将实现留给了java,而是交给了jvm去加载。
主要是先获取 ClassLoader, 然后调用 native 方法，获取信息，加载类则是回调 java.lang.ClassLoader.
最后，jvm又会回调 ClassLoader 进类加载。
//
public Class<?> loadClass(String name) throws ClassNotFoundException {
return loadClass(name, false);
}
// sun.misc.Launcher
public Class<?> loadClass(String var1, boolean var2) throws ClassNotFoundException {
int var3 = var1.lastIndexOf(46);
if(var3 != -1) {
SecurityManager var4 = System.getSecurityManager();
if(var4 != null) {
var4.checkPackageAccess(var1.substring(0, var3));
}
}
if(this.ucp.knownToNotExist(var1)) {
Class var5 = this.findLoadedClass(var1);
if(var5 != null) {
if(var2) {
this.resolveClass(var5);
}
return var5;
} else {
throw new ClassNotFoundException(var1);
}
} else {
return super.loadClass(var1, var2);
}
}
// java.lang.ClassLoader
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
// 先获取锁
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
// 如果已经加载了的话，就不用再加载了
Class<?> c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
try {
// 双亲委托加载
if (parent != null) {
c = parent.loadClass(name, false);
} else {
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
}
// 父类没有加载到时，再自己加载
if (c == null) {
// If still not found, then invoke findClass in order
// to find the class.
long t1 = System.nanoTime();
c = findClass(name);
// this is the defining class loader; record the stats
sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
sun.misc.PerfCounter.getFindClasses().increment();
}
}
if (resolve) {
resolveClass(c);
}
return c;
}
}
protected Object getClassLoadingLock(String className) {
Object lock = this;
if (parallelLockMap != null) {
// 使用 ConcurrentHashMap来保存锁
Object newLock = new Object();
lock = parallelLockMap.putIfAbsent(className, newLock);
if (lock == null) {
lock = newLock;
}
}
return lock;
}
protected final Class<?> findLoadedClass(String name) {
if (!checkName(name))
return null;
return findLoadedClass0(name);
}
下面来看一下 newInstance() 的实现方式。
// 首先肯定是 Class.newInstance
@CallerSensitive
public T newInstance()
throws InstantiationException, IllegalAccessException {
if (System.getSecurityManager() != null) {
checkMemberAccess(Member.PUBLIC, Reflection.getCallerClass(), false);
}
// NOTE: the following code may not be strictly correct under
// the current Java memory model.
// Constructor lookup
// newInstance() 其实相当于调用类的无参构造函数，所以，首先要找到其无参构造器
if (cachedConstructor == null) {
if (this == Class.class) {
// 不允许调用 Class 的 newInstance() 方法
throw new IllegalAccessException(
"Can not call newInstance() on the Class for java.lang.Class"
);
}
try {
// 获取无参构造器
Class<?>[] empty = {};
final Constructor<T> c = getConstructor0(empty, Member.DECLARED);
// Disable accessibility checks on the constructor
// since we have to do the security check here anyway
// (the stack depth is wrong for the Constructor's
// security check to work)
java.security.AccessController.doPrivileged(
new java.security.PrivilegedAction<Void>() {
public Void run() {
c.setAccessible(true);
return null;
}
});
cachedConstructor = c;
} catch (NoSuchMethodException e) {
throw (InstantiationException)
new InstantiationException(getName()).initCause(e);
}
}
Constructor<T> tmpConstructor = cachedConstructor;
// Security check (same as in java.lang.reflect.Constructor)
int modifiers = tmpConstructor.getModifiers();
if (!Reflection.quickCheckMemberAccess(this, modifiers)) {
Class<?> caller = Reflection.getCallerClass();
if (newInstanceCallerCache != caller) {
Reflection.ensureMemberAccess(caller, this, null, modifiers);
newInstanceCallerCache = caller;
}
}
// Run constructor
try {
// 调用无参构造器
return tmpConstructor.newInstance((Object[]) null);
} catch (InvocationTargetException e) {
Unsafe.getUnsafe().throwException(e.getTargetException());
// Not reached
return null;
}
}
newInstance() 主要做了三件事：
- 权限检测，如果不通过直接抛出异常；查找无参构造器，并将其缓存起来；调用具体方法的无参构造方法，生成实例并返回；
下面是获取构造器的过程：
private Constructor<T> getConstructor0(Class<?>[] parameterTypes,
int which) throws NoSuchMethodException
{
// 获取所有构造器
Constructor<T>[] constructors = privateGetDeclaredConstructors((which == Member.PUBLIC));
for (Constructor<T> constructor : constructors) {
if (arrayContentsEq(parameterTypes,
constructor.getParameterTypes())) {
return getReflectionFactory().copyConstructor(constructor);
}
}
throw new NoSuchMethodException(getName() ".<init>" argumentTypesToString(parameterTypes));
}
getConstructor0() 为获取匹配的构造方器；分三步：
- 先获取所有的constructors, 然后通过进行参数类型比较；找到匹配后，通过 ReflectionFactory copy一份constructor返回；否则抛出 NoSuchMethodException;
// 获取当前类所有的构造方法，通过jvm或者缓存
// Returns an array of "root" constructors. These Constructor
// objects must NOT be propagated to the outside world, but must
// instead be copied via ReflectionFactory.copyConstructor.
private Constructor<T>[] privateGetDeclaredConstructors(boolean publicOnly) {
checkInitted();
Constructor<T>[] res;
// 调用 reflectionData(), 获取保存的信息，使用软引用保存，从而使内存不够可以回收
ReflectionData<T> rd = reflectionData();
if (rd != null) {
res = publicOnly ? rd.publicConstructors : rd.declaredConstructors;
// 存在缓存，则直接返回
if (res != null) return res;
}
// No cached value available; request value from VM
if (isInterface()) {
@SuppressWarnings("unchecked")
Constructor<T>[] temporaryRes = (Constructor<T>[]) new Constructor<?>[0];
res = temporaryRes;
} else {
// 使用native方法从jvm获取构造器
res = getDeclaredConstructors0(publicOnly);
}
if (rd != null) {
// 最后，将从jvm中读取的内容，存入缓存
if (publicOnly) {
rd.publicConstructors = res;
} else {
rd.declaredConstructors = res;
}
}
return res;
}
// Lazily create and cache ReflectionData
private ReflectionData<T> reflectionData() {
SoftReference<ReflectionData<T>> reflectionData = this.reflectionData;
int classRedefinedCount = this.classRedefinedCount;
ReflectionData<T> rd;
if (useCaches &&
reflectionData != null &&
(rd = reflectionData.get()) != null &&
rd.redefinedCount == classRedefinedCount) {
return rd;
}
// else no SoftReference or cleared SoftReference or stale ReflectionData
// -> create and replace new instance
return newReflectionData(reflectionData, classRedefinedCount);
}
// 新创建缓存，保存反射信息
private ReflectionData<T> newReflectionData(SoftReference<ReflectionData<T>> oldReflectionData,
int classRedefinedCount) {
if (!useCaches) return null;
// 使用cas保证更新的线程安全性，所以反射是保证线程安全的
while (true) {
ReflectionData<T> rd = new ReflectionData<>(classRedefinedCount);
// try to CAS it...
if (Atomic.casReflectionData(this, oldReflectionData, new SoftReference<>(rd))) {
return rd;
}
// 先使用CAS更新，如果更新成功，则立即返回，否则测查当前已被其他线程更新的情况，如果和自己想要更新的状态一致，则也算是成功了
oldReflectionData = this.reflectionData;
classRedefinedCount = this.classRedefinedCount;
if (oldReflectionData != null &&
(rd = oldReflectionData.get()) != null &&
rd.redefinedCount == classRedefinedCount) {
return rd;
}
}
}
如上，privateGetDeclaredConstructors(), 获取所有的构造器主要步骤；
- 先尝试从缓存中获取；如果缓存没有，则从jvm中重新获取，并存入缓存，缓存使用软引用进行保存，保证内存可用；
另外，使用 relactionData() 进行缓存保存；ReflectionData 的数据结构如下。
// reflection data that might get invalidated when JVM TI RedefineClasses() is called
private static class ReflectionData<T> {
volatile Field[] declaredFields;
volatile Field[] publicFields;
volatile Method[] declaredMethods;
volatile Method[] publicMethods;
volatile Constructor<T>[] declaredConstructors;
volatile Constructor<T>[] publicConstructors;
// Intermediate results for getFields and getMethods
volatile Field[] declaredPublicFields;
volatile Method[] declaredPublicMethods;
volatile Class<?>[] interfaces;
// Value of classRedefinedCount when we created this ReflectionData instance
final int redefinedCount;
ReflectionData(int redefinedCount) {
this.redefinedCount = redefinedCount;
}
}
其中，还有一个点，就是如何比较构造是否是要查找构造器，其实就是比较类型完成相等就完了，有一个不相等则返回false。
private static boolean arrayContentsEq(Object[] a1, Object[] a2) {
if (a1 == null) {
return a2 == null || a2.length == 0;
}
if (a2 == null) {
return a1.length == 0;
}
if (a1.length != a2.length) {
return false;
}
for (int i = 0; i < a1.length; i ) {
if (a1[i] != a2[i]) {
return false;
}
}
return true;
}
// sun.reflect.ReflectionFactory
/**
* Makes a copy of the passed constructor. The returned
* <p>
* constructor is a "child" of the passed one; see the comments
* <p>
* in Constructor.java for details.
*/
public <T> Constructor<T> copyConstructor(Constructor<T> arg) {
return langReflectAccess().copyConstructor(arg);
}
// java.lang.reflect.Constructor, copy 其实就是新new一个 Constructor 出来
Constructor<T> copy() {
// This routine enables sharing of ConstructorAccessor objects
// among Constructor objects which refer to the same underlying
// method in the VM. (All of this contortion is only necessary
// because of the "accessibility" bit in AccessibleObject,
// which implicitly requires that new java.lang.reflect
// objects be fabricated for each reflective call on Class
// objects.)
if (this.root != null)
throw new IllegalArgumentException("Can not copy a non-root Constructor");
Constructor<T> res = new Constructor<>(clazz,
parameterTypes,
exceptionTypes, modifiers, slot,
signature,
annotations,
parameterAnnotations);
// root 指向当前 constructor
res.root = this;
// Might as well eagerly propagate this if already present
res.constructorAccessor = constructorAccessor;
return res;
}
通过上面，获取到 Constructor 了。
接下来就只需调用其相应构造器的 newInstance()，即返回实例了。
// return tmpConstructor.newInstance((Object[])null);
// java.lang.reflect.Constructor
@CallerSensitive
public T newInstance(Object... initargs)
throws InstantiationException, IllegalAccessException,
IllegalArgumentException, InvocationTargetException {
if (!override) {
if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) {
Class<?> caller = Reflection.getCallerClass();
checkAccess(caller, clazz, null, 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;
}
// sun.reflect.DelegatingConstructorAccessorImpl
public Object newInstance(Object[] args)
throws InstantiationException,
IllegalArgumentException,
InvocationTargetException {
return delegate.newInstance(args);
}
// sun.reflect.NativeConstructorAccessorImpl
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);
}
// 调用native方法，进行调用 constructor
return newInstance0(c, args);
}
返回构造器的实例后，可以根据外部进行进行类型转换，从而使用接口或方法进行调用实例功能了。
反射获取方法
- 第一步，先获取 Method;
// java.lang.Class
@CallerSensitive
public Method getDeclaredMethod(String name, Class<?>... parameterTypes)
throws NoSuchMethodException, SecurityException {
checkMemberAccess(Member.DECLARED, Reflection.getCallerClass(), true);
Method method = searchMethods(privateGetDeclaredMethods(false), name, parameterTypes);
if (method == null) {
throw new NoSuchMethodException(getName() "." name argumentTypesToString(parameterTypes));
}
return method;
}
忽略第一个检查权限，剩下就只有两个动作了。
- 获取所有方法列表；根据方法名称和方法列表，选出符合要求的方法；如果没有找到相应方法，抛出异常，否则返回对应方法；
所以，先看一下怎样获取类声明的所有方法？
// Returns an array of "root" methods. These Method objects must NOT
// be propagated to the outside world, but must instead be copied
// via ReflectionFactory.copyMethod.
private Method[] privateGetDeclaredMethods(boolean publicOnly) {
checkInitted();
Method[] res;
ReflectionData<T> rd = reflectionData();
if (rd != null) {
res = publicOnly ? rd.declaredPublicMethods : rd.declaredMethods;
if (res != null) return res;
}
// No cached value available; request value from VM
res = Reflection.filterMethods(this, getDeclaredMethods0(publicOnly));
if (rd != null) {
if (publicOnly) {
rd.declaredPublicMethods = res;
} else {
rd.declaredMethods = res;
}
}
return res;
}
很相似，和获取所有构造器的方法很相似，都是先从缓存中获取方法，如果没有，则从jvm中获取。
不同的是，方法列表需要进行过滤 Reflection.filterMethods;当然后面看来，这个方法我们一般不会派上用场。
// sun.misc.Reflection
public static Method[] filterMethods(Class<?> containingClass, Method[] methods) {
if (methodFilterMap == null) {
// Bootstrapping
return methods;
}
return (Method[]) filter(methods, methodFilterMap.get(containingClass));
}
// 可以过滤指定的方法，一般为空，如果要指定过滤，可以调用 registerMethodsToFilter(), 或者...
private static Member[] filter(Member[] members, String[] filteredNames) {
if ((filteredNames == null) || (members.length == 0)) {
return members;
}
int numNewMembers = 0;
for (Member member : members) {
boolean shouldSkip = false;
for (String filteredName : filteredNames) {
if (member.getName() == filteredName) {
shouldSkip = true;
break;
}
}
if (!shouldSkip) {
numNewMembers;
}
}
Member[] newMembers =
(Member[]) Array.newInstance(members[0].getClass(), numNewMembers);
int destIdx = 0;
for (Member member : members) {
boolean shouldSkip = false;
for (String filteredName : filteredNames) {
if (member.getName() == filteredName) {
shouldSkip = true;
break;
}
}
if (!shouldSkip) {
newMembers[destIdx ] = member;
}
}
return newMembers;
}
- 第二步，根据方法名和参数类型过滤指定方法返回：
private static Method searchMethods(Method[] methods,
String name,
Class<?>[] parameterTypes) {
Method res = null;
// 使用常量池，避免重复创建String
String internedName = name.intern();
for (int i = 0; i < methods.length; i ) {
Method m = methods[i];
if (m.getName() == internedName
&& arrayContentsEq(parameterTypes, m.getParameterTypes())
&& (res == null
|| res.getReturnType().isAssignableFrom(m.getReturnType())))
res = m;
}
return (res == null ? res : getReflectionFactory().copyMethod(res));
}
大概意思看得明白，就是匹配到方法名，然后参数类型匹配，才可以。
- 但是可以看到，匹配到一个方法，并没有退出for循环，而是继续进行匹配。
- 这里是匹配最精确的子类进行返回（最优匹配）
- 最后，还是通过 ReflectionFactory, copy 方法后返回。
调用 method.invoke() 方法
@CallerSensitive
public Object invoke(Object obj, Object... args)
throws IllegalAccessException, IllegalArgumentException,
InvocationTargetException
{
if (!override) {
if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) {
Class<?> caller = Reflection.getCallerClass();
checkAccess(caller, clazz, obj, modifiers);
}
}
MethodAccessor ma = methodAccessor; // read volatile
if (ma == null) {
ma = acquireMethodAccessor();
}
return ma.invoke(obj, args);
}
invoke时，是通过 MethodAccessor 进行调用的，而 MethodAccessor 是个接口，在第一次时调用 acquireMethodAccessor() 进行新创建。
// probably make the implementation more scalable.
private MethodAccessor acquireMethodAccessor() {
// First check to see if one has been created yet, and take it
// if so
MethodAccessor tmp = null;
if (root != null) tmp = root.getMethodAccessor();
if (tmp != null) {
// 存在缓存时，存入 methodAccessor，否则调用 ReflectionFactory 创建新的 MethodAccessor
methodAccessor = tmp;
} else {
// Otherwise fabricate one and propagate it up to the root
tmp = reflectionFactory.newMethodAccessor(this);
setMethodAccessor(tmp);
}
return tmp;
}
// sun.reflect.ReflectionFactory
public MethodAccessor newMethodAccessor(Method method) {
checkInitted();
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;
}
}
两个Accessor详情：
// NativeMethodAccessorImpl / DelegatingMethodAccessorImplclass 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);
}
class DelegatingMethodAccessorImpl extends MethodAccessorImpl {
private MethodAccessorImpl delegate;
DelegatingMethodAccessorImpl(MethodAccessorImpl delegate) {
setDelegate(delegate);
}
public Object invoke(Object obj, Object[] args)
throws IllegalArgumentException, InvocationTargetException {
return delegate.invoke(obj, args);
}
void setDelegate(MethodAccessorImpl delegate) {
this.delegate = delegate;
}
进行 ma.invoke(obj, args); 调用时，调用 DelegatingMethodAccessorImpl.invoke();
最后被委托到 NativeMethodAccessorImpl.invoke(), 即：
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);
}
// invoke0 是个 native 方法，由jvm进行调用业务方法。从而完成反射调用功能。
return invoke0(method, obj, args);
}
其中， generateMethod() 是生成具体类的方法：
/**
* This routine is not thread-safe
*/
public MethodAccessor generateMethod(Class<?> declaringClass,
String name,
Class<?>[] parameterTypes,
Class<?> returnType,
Class<?>[] checkedExceptions,
int modifiers) {
return (MethodAccessor) generate(declaringClass,
name,
parameterTypes,
returnType,
checkedExceptions,
modifiers,
false,
false,
null);
}
generate() 戳详情。
/**
* This routine is not thread-safe
*/
private MagicAccessorImpl generate(final Class<?> declaringClass,
String name,
Class<?>[]parameterTypes,
Class<?> returnType,
Class<?>[]checkedExceptions,
int modifiers,
boolean isConstructor,
boolean forSerialization,
Class<?> serializationTargetClass)
{
ByteVector vec=ByteVectorFactory.create();
asm=new ClassFileAssembler(vec);
this.declaringClass=declaringClass;
this.parameterTypes=parameterTypes;
this.returnType=returnType;
this.modifiers=modifiers;
this.isConstructor=isConstructor;
this.forSerialization=forSerialization;
asm.emitMagicAndVersion();
// Constant pool entries:
// ( * = Boxing information: optional)
// ( = Shared entries provided by AccessorGenerator)
// (^ = Only present if generating SerializationConstructorAccessor)
// [UTF-8] [This class's name]
// [CONSTANT_Class_info] for above
// [UTF-8] "sun/reflect/{MethodAccessorImpl,ConstructorAccessorImpl,SerializationConstructorAccessorImpl}"
// [CONSTANT_Class_info] for above
// [UTF-8] [Target class's name]
// [CONSTANT_Class_info] for above
// ^ [UTF-8] [Serialization: Class's name in which to invoke constructor]
// ^ [CONSTANT_Class_info] for above
// [UTF-8] target method or constructor name
// [UTF-8] target method or constructor signature
// [CONSTANT_NameAndType_info] for above
// [CONSTANT_Methodref_info or CONSTANT_InterfaceMethodref_info] for target method
// [UTF-8] "invoke" or "newInstance"
// [UTF-8] invoke or newInstance descriptor
// [UTF-8] descriptor for type of non-primitive parameter 1
// [CONSTANT_Class_info] for type of non-primitive parameter 1
// ...
// [UTF-8] descriptor for type of non-primitive parameter n
// [CONSTANT_Class_info] for type of non-primitive parameter n
// [UTF-8] "java/lang/Exception"
// [CONSTANT_Class_info] for above
// [UTF-8] "java/lang/ClassCastException"
// [CONSTANT_Class_info] for above
// [UTF-8] "java/lang/NullPointerException"
// [CONSTANT_Class_info] for above
// [UTF-8] "java/lang/IllegalArgumentException"
// [CONSTANT_Class_info] for above
// [UTF-8] "java/lang/InvocationTargetException"
// [CONSTANT_Class_info] for above
// [UTF-8] "<init>"
// [UTF-8] "()V"
// [CONSTANT_NameAndType_info] for above
// [CONSTANT_Methodref_info] for NullPointerException's constructor
// [CONSTANT_Methodref_info] for IllegalArgumentException's constructor
// [UTF-8] "(Ljava/lang/String;)V"
// [CONSTANT_NameAndType_info] for "<init>(Ljava/lang/String;)V"
// [CONSTANT_Methodref_info] for IllegalArgumentException's constructor taking a String
// [UTF-8] "(Ljava/lang/Throwable;)V"
// [CONSTANT_NameAndType_info] for "<init>(Ljava/lang/Throwable;)V"
// [CONSTANT_Methodref_info] for InvocationTargetException's constructor
// [CONSTANT_Methodref_info] for "super()"
// [UTF-8] "java/lang/Object"
// [CONSTANT_Class_info] for above
// [UTF-8] "toString"
// [UTF-8] "()Ljava/lang/String;"
// [CONSTANT_NameAndType_info] for "toString()Ljava/lang/String;"
// [CONSTANT_Methodref_info] for Object's toString method
// [UTF-8] "Code"
// [UTF-8] "Exceptions"
// * [UTF-8] "java/lang/Boolean"
// * [CONSTANT_Class_info] for above
// * [UTF-8] "(Z)V"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
// * [UTF-8] "booleanValue"
// * [UTF-8] "()Z"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
// * [UTF-8] "java/lang/Byte"
// * [CONSTANT_Class_info] for above
// * [UTF-8] "(B)V"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
// * [UTF-8] "byteValue"
// * [UTF-8] "()B"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
// * [UTF-8] "java/lang/Character"
// * [CONSTANT_Class_info] for above
// * [UTF-8] "(C)V"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
// * [UTF-8] "charValue"
// * [UTF-8] "()C"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
// * [UTF-8] "java/lang/Double"
// * [CONSTANT_Class_info] for above
// * [UTF-8] "(D)V"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
// * [UTF-8] "doubleValue"
// * [UTF-8] "()D"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
// * [UTF-8] "java/lang/Float"
// * [CONSTANT_Class_info] for above
// * [UTF-8] "(F)V"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
// * [UTF-8] "floatValue"
// * [UTF-8] "()F"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
// * [UTF-8] "java/lang/Integer"
// * [CONSTANT_Class_info] for above
// * [UTF-8] "(I)V"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
// * [UTF-8] "intValue"
// * [UTF-8] "()I"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
// * [UTF-8] "java/lang/Long"
// * [CONSTANT_Class_info] for above
// * [UTF-8] "(J)V"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
// * [UTF-8] "longValue"
// * [UTF-8] "()J"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
// * [UTF-8] "java/lang/Short"
// * [CONSTANT_Class_info] for above
// * [UTF-8] "(S)V"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
// * [UTF-8] "shortValue"
// * [UTF-8] "()S"
// * [CONSTANT_NameAndType_info] for above
// * [CONSTANT_Methodref_info] for above
short numCPEntries=NUM_BASE_CPOOL_ENTRIES NUM_COMMON_CPOOL_ENTRIES;
boolean usesPrimitives=usesPrimitiveTypes();
if(usesPrimitives){
numCPEntries =NUM_BOXING_CPOOL_ENTRIES;
}
if(forSerialization){
numCPEntries =NUM_SERIALIZATION_CPOOL_ENTRIES;
}
// Add in variable-length number of entries to be able to describe
// non-primitive parameter types and checked exceptions.
numCPEntries =(short)(2*numNonPrimitiveParameterTypes());
asm.emitShort(add(numCPEntries,S1));
final String generatedName=generateName(isConstructor,forSerialization);
asm.emitConstantPoolUTF8(generatedName);
asm.emitConstantPoolClass(asm.cpi());
thisClass=asm.cpi();
if(isConstructor){
if(forSerialization){
asm.emitConstantPoolUTF8
("sun/reflect/SerializationConstructorAccessorImpl");
}else{
asm.emitConstantPoolUTF8("sun/reflect/ConstructorAccessorImpl");
}
}else{
asm.emitConstantPoolUTF8("sun/reflect/MethodAccessorImpl");
}
asm.emitConstantPoolClass(asm.cpi());
superClass=asm.cpi();
asm.emitConstantPoolUTF8(getClassName(declaringClass,false));
asm.emitConstantPoolClass(asm.cpi());
targetClass=asm.cpi();
short serializationTargetClassIdx=(short)0;
if(forSerialization){
asm.emitConstantPoolUTF8(getClassName(serializationTargetClass,false));
asm.emitConstantPoolClass(asm.cpi());
serializationTargetClassIdx=asm.cpi();
}
asm.emitConstantPoolUTF8(name);
asm.emitConstantPoolUTF8(buildInternalSignature());
asm.emitConstantPoolNameAndType(sub(asm.cpi(),S1),asm.cpi());
if(isInterface()){
asm.emitConstantPoolInterfaceMethodref(targetClass,asm.cpi());
}else{
if(forSerialization){
asm.emitConstantPoolMethodref(serializationTargetClassIdx,asm.cpi());
}else{
asm.emitConstantPoolMethodref(targetClass,asm.cpi());
}
}
targetMethodRef=asm.cpi();
if(isConstructor){
asm.emitConstantPoolUTF8("newInstance");
}else{
asm.emitConstantPoolUTF8("invoke");
}
invokeIdx=asm.cpi();
if(isConstructor){
asm.emitConstantPoolUTF8("([Ljava/lang/Object;)Ljava/lang/Object;");
}else{
asm.emitConstantPoolUTF8
("(Ljava/lang/Object;[Ljava/lang/Object;)Ljava/lang/Object;");
}
invokeDescriptorIdx=asm.cpi();
// Output class information for non-primitive parameter types
nonPrimitiveParametersBaseIdx=add(asm.cpi(),S2);
for(int i=0;i<parameterTypes.length;i ){
Class<?> c=parameterTypes[i];
if(!isPrimitive(c)){
asm.emitConstantPoolUTF8(getClassName(c,false));
asm.emitConstantPoolClass(asm.cpi());
}
}
// Entries common to FieldAccessor, MethodAccessor and ConstructorAccessor
emitCommonConstantPoolEntries();
// Boxing entries
if(usesPrimitives){
emitBoxingContantPoolEntries();
}
if(asm.cpi()!=numCPEntries){
throw new InternalError("Adjust this code (cpi = " asm.cpi()
", numCPEntries = " numCPEntries ")");
}
// Access flags
asm.emitShort(ACC_PUBLIC);
// This class
asm.emitShort(thisClass);
// Superclass
asm.emitShort(superClass);
// Interfaces count and interfaces
asm.emitShort(S0);
// Fields count and fields
asm.emitShort(S0);
// Methods count and methods
asm.emitShort(NUM_METHODS);
emitConstructor();
emitInvoke();
// Additional attributes (none)
asm.emitShort(S0);
// Load class
vec.trim();
final byte[]bytes=vec.getData();
// Note: the class loader is the only thing that really matters
// here -- it's important to get the generated code into the
// same namespace as the target class. Since the generated code
// is privileged anyway, the protection domain probably doesn't
// matter.
return AccessController.doPrivileged(
new PrivilegedAction<MagicAccessorImpl>(){
public MagicAccessorImpl run(){
try{
return(MagicAccessorImpl)
ClassDefiner.defineClass
(generatedName,
bytes,
0,
bytes.length,
declaringClass.getClassLoader()).newInstance();
}catch(InstantiationException|IllegalAccessException e){
throw new InternalError(e);
}
}
});
}
咱们主要看这一句：ClassDefiner.defineClass(xx, declaringClass.getClassLoader()).newInstance();
在ClassDefiner.defineClass方法实现中，每被调用一次都会生成一个DelegatingClassLoader类加载器对象 ，这里每次都生成新的类加载器，是为了性能考虑，在某些情况下可以卸载这些生成的类，因为类的卸载是只有在类加载器可以被回收的情况下才会被回收的，如果用了原来的类加载器，那可能导致这些新创建的类一直无法被卸载。
而反射生成的类，有时候可能用了就可以卸载了，所以使用其独立的类加载器，从而使得更容易控制反射类的生命周期。
反射调用流程小结
最后，用几句话总结反射的实现原理：
反射类及反射方法的获取，都是通过从列表中搜寻查找匹配的方法，所以查找性能会随类的大小方法多少而变化；
每个类都会有一个与之对应的Class实例，从而每个类都可以获取method反射方法，并作用到其他实例身上；
反射也是考虑了线程安全的，放心使用；
反射使用软引用relectionData缓存class信息，避免每次重新从jvm获取带来的开销；
反射调用多次生成新代理Accessor, 而通过字节码生存的则考虑了卸载功能，所以会使用独立的类加载器；
当找到需要的方法，都会copy一份出来，而不是使用原来的实例，从而保证数据隔离；
调度反射方法，最终是由jvm执行invoke0()执行
