反射是java的一個特性，這一特性也使得它給了廣大的第三方架構和開發過者很大的想像空間。
通過反射，java可以動态的加載未知的外部配置對象，臨時生成位元組碼進行加載使用，進而使代碼更靈活！可以極大地提高應用的擴充性！
但是，除了停留在使用其華麗功能，我們還可以去看看其實作！
主要看兩個方法的使用:
來個例子！
public class HelloReflect {
public static void main(String[] args) {
try {
// 1. 使用外部配置的實作，進行動态加載類
TempFunctionTest test = (TempFunctionTest)Class.forName("com.tester.HelloReflect").newInstance();
test.sayHello("call directly");
// 2. 根據配置的函數名，進行方法調用（不需要通用的接口抽象）
Object t2 = new TempFunctionTest();
Method method = t2.getClass().getDeclaredMethod("sayHello", String.class);
method.invoke(test, "method invoke");
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (NoSuchMethodException e ) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
}
public void sayHello(String word) {
System.out.println("hello," + word);
}
} 運作結果顯而易見！我們來看執行流程！
1. 反射擷取類執行個體 Class.forName("C.a.xxx");
首先調用了 java.lang.Class 的靜态方法，擷取類資訊！
@CallerSensitive
public static Class<?> forName(String className)
throws ClassNotFoundException {
// 先通過反射，擷取調用進來的類資訊，進而擷取目前的 classLoader
Class<?> caller = Reflection.getCallerClass();
// 調用native方法進行擷取class資訊
return forName0(className, true, ClassLoader.getClassLoader(caller), caller);
} forName()反射擷取類資訊，并沒有将實作留給了java,而是交給了jvm去加載！
主要是先擷取 ClassLoader, 然後調用 native 方法，擷取資訊，加載類則是回調 java.lang.ClassLoader.
最後，jvm又會回調 ClassLoader 進類加載！
//
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
{
// 先擷取鎖
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
// 如果已經加載了的話，就不用再加載了
Class<?> c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
try {
// 雙親委托加載
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
}
// 父類沒有加載到時，再自己加載
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來儲存鎖
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);
} 下面來看一下 newInstance() 的實作方式！
// 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() 其實相當于調用類的無參構造函數，是以，首先要找到其無參構造器
if (cachedConstructor == null) {
if (this == Class.class) {
// 不允許調用 Class 的 newInstance() 方法
throw new IllegalAccessException(
"Can not call newInstance() on the Class for java.lang.Class"
);
}
try {
// 擷取無參構造器
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 {
// 調用無參構造器
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：
1. 權限檢測，如果不通過直接抛出異常；
2. 查找無參構造器，并将其緩存起來；
3. 調用具體方法的無參構造方法，生成執行個體并傳回；
下面是擷取構造器的過程：
private Constructor<T> getConstructor0(Class<?>[] parameterTypes,
int which) throws NoSuchMethodException
{
// 擷取所有構造器
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() 為擷取比對的構造方器；分三步：
1. 先擷取所有的constructors, 然後通過進行參數類型比較；
2. 找到比對後，通過 ReflectionFactory copy一份constructor傳回；
3. 否則抛出 NoSuchMethodException;
// 擷取目前類所有的構造方法，通過jvm或者緩存
// 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;
// 調用 reflectionData(), 擷取儲存的資訊，使用軟引用儲存，進而使記憶體不夠可以回收
ReflectionData<T> rd = reflectionData();
if (rd != null) {
res = publicOnly ? rd.publicConstructors : rd.declaredConstructors;
// 存在緩存，則直接傳回
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方法從jvm擷取構造器
res = getDeclaredConstructors0(publicOnly);
}
if (rd != null) {
// 最後，将從jvm中讀取的内容，存入緩存
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);
}
// 新建立緩存，儲存反射資訊
private ReflectionData<T> newReflectionData(SoftReference<ReflectionData<T>> oldReflectionData,
int classRedefinedCount) {
if (!useCaches) return null;
// 使用cas保證更新的線程安全性，是以反射是保證線程安全的
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，則立即傳回，否則測查目前已被其他線程更新的情況，如果和自己想要更新的狀态一緻，則也算是成功了
oldReflectionData = this.reflectionData;
classRedefinedCount = this.classRedefinedCount;
if (oldReflectionData != null &&
(rd = oldReflectionData.get()) != null &&
rd.redefinedCount == classRedefinedCount) {
return rd;
}
}
} 如上，privateGetDeclaredConstructors(), 擷取所有的構造器主要步驟；
1. 先嘗試從緩存中擷取；
2. 如果緩存沒有，則從jvm中重新擷取，并存入緩存，緩存使用軟引用進行儲存，保證記憶體可用；
另外，使用 relactionData() 進行緩存儲存；ReflectionData 的資料結構如下！
// 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，還有一個點，就是如何比較構造是否是要查找構造器，其實就是比較類型完成相等就完了，有一個不相等則傳回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
constructor is a "child" of the passed one; see the comments
in Constructor.java for details. */
public <T> Constructor<T> copyConstructor(Constructor<T> arg) {
return langReflectAccess().copyConstructor(arg);
}
// java.lang.reflect.Constructor, copy 其實就是新new一個 Constructor 出來
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 指向目前 constructor
res.root = this;
// Might as well eagerly propagate this if already present
res.constructorAccessor = constructorAccessor;
return res;
}
通過上面，擷取到 Constructor 了！
接下來就隻需調用其相應構造器的 newInstance()，即傳回執行個體了！
// 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);
}
// 調用native方法，進行調用 constructor
return newInstance0(c, args);
} 傳回構造器的執行個體後，可以根據外部進行進行類型轉換，進而使用接口或方法進行調用執行個體功能了。
2. 反射擷取方法 c.class.getDeclaredMethod();Method.invoke() 反射調用方法！
第一步，先擷取 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;
} 忽略第一個檢查權限，剩下就隻有兩個動作了！
1. 擷取所有方法清單；
2. 根據方法名稱和方法清單，選出符合要求的方法；
3. 如果沒有找到相應方法，抛出異常，否則傳回對應方法；
是以，先看一下怎樣擷取類聲明的所有方法？
// 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，和擷取所有構造器的方法很相似，都是先從緩存中擷取方法，如果沒有，則從jvm中擷取！
不同的是，方法清單需要進行過濾 Reflection.filterMethods;當然後面看來，這個方法我們一般不會派上用場！
// sun.misc.Reflection
public static Method[] filterMethods(Class<?> containingClass, Method[] methods) {
if (methodFilterMap == null) {
// Bootstrapping
return methods;
}
return (Method[])filter(methods, methodFilterMap.get(containingClass));
}
// 可以過濾指定的方法，一般為空，如果要指定過濾，可以調用 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，根據方法名和參數類型過濾指定方法傳回：
private static Method searchMethods(Method[] methods,
String name,
Class<?>[] parameterTypes)
{
Method res = null;
// 使用常量池，避免重複建立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，就是比對到方法名，然後參數類型比對，才可以！
但是，可以看到，比對到一個方法，并沒有退出for循環，而是繼續進行比對！
這裡，是比對最精确的子類進行傳回（最優比對）
最後，還是通過 ReflectionFactory, copy 方法後傳回！
第三步，調用 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時，是通過 MethodAccessor 進行調用的，而 MethodAccessor 是個接口，在第一次時調用 acquireMethodAccessor() 進行新建立！
// 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) {
// 存在緩存時，存入 methodAccessor，否則調用 ReflectionFactory 建立新的 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;
}
} 兩個Accessor詳情：
// NativeMethodAccessorImpl / DelegatingMethodAccessorImpl
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);
}
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;
}
} View Code
進行 ma.invoke(obj, args); 調用時，調用 DelegatingMethodAccessorImpl.invoke();
最後被委托到 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 是個 native 方法，由jvm進行調用業務方法！進而完成反射調用功能！
return invoke0(method, obj, args);
}
其中， generateMethod() 是生成具體類的方法：
/** 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() 戳詳情！
/** 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);
}
}
});
} 咱們主要看這一句：ClassDefiner.defineClass(xx, declaringClass.getClassLoader()).newInstance();
在ClassDefiner.defineClass方法實作中，每被調用一次都會生成一個DelegatingClassLoader類加載器對象 ，這裡每次都生成新的類加載器，是為了性能考慮，在某些情況下可以解除安裝這些生成的類，因為類的解除安裝是隻有在類加載器可以被回收的情況下才會被回收的，如果用了原來的類加載器，那可能導緻這些新建立的類一直無法被解除安裝！
而反射生成的類，有時候可能用了就可以解除安裝了，是以使用其獨立的類加載器，進而使得更容易控制反射類的生命周期！
最後，用幾句話總結反射的實作原理：
1. 反射類及反射方法的擷取，都是通過從清單中搜尋查找比對的方法，是以查找性能會随類的大小方法多少而變化；
2. 每個類都會有一個與之對應的Class執行個體，進而每個類都可以擷取method反射方法，并作用到其他執行個體身上；
3. 反射也是考慮了線程安全的，放心使用；
4. 反射使用軟引用relectionData緩存class資訊，避免每次重新從jvm擷取帶來的開銷；
5. 反射調用多次生成新代理Accessor, 而通過位元組碼生存的則考慮了解除安裝功能，是以會使用獨立的類加載器；
6. 當找到需要的方法，都會copy一份出來，而不是使用原來的執行個體，進而保證資料隔離；
7. 排程反射方法，最終是由jvm執行invoke0()執行；
不要害怕今日的苦，你要相信明天，更苦！