public class A {
public void foo(String name) {
System.out.println("Hello, " + name);
}
}
可以編寫另外一個類來反射調用A上的方法：
import java.lang.reflect.Method;
public class TestClassLoad {
public static void main(String[] args) throws Exception {
Class<?> clz = Class.forName("A");
Object o = clz.newInstance();
Method m = clz.getMethod("foo", String.class);
for (int i = 0; i < 16; i++) {
m.invoke(o, Integer.toString(i));
}
}
}
注意到TestClassLoad類上不會有對類A的符號依賴——也就是說在加載並初始化TestClassLoad類時不需要關心類A的存在與否，而是等到main()方法執行到調用Class.forName()時才試圖對類A做動態加載；這裏用的是一個參數版的forName()，也就是使用當前方法所在類的ClassLoader來加載，並且初始化新加載的類。……好吧這個細節跟主題沒啥關係。
回到主題。這次我的測試環境是Sun的JDK 1.6.0 update 13 build 03。編譯上述代碼，並在執行TestClassLoad時加入-XX:+TraceClassLoading參數（或者-verbose:class或者直接-verbose都行），如下：
[Loaded TestClassLoad from file:/D:/temp_code/test_java_classload/] [Loaded A from file:/D:/temp_code/test_java_classload/] [Loaded sun.reflect.NativeMethodAccessorImpl from shared objects file] [Loaded sun.reflect.DelegatingMethodAccessorImpl from shared objects file] Hello, 0 Hello, 1 Hello, 2 Hello, 3 Hello, 4 Hello, 5 Hello, 6 Hello, 7 Hello, 8 Hello, 9 Hello, 10 Hello, 11 Hello, 12 Hello, 13 Hello, 14 [Loaded sun.reflect.ClassFileConstants from shared objects file] [Loaded sun.reflect.AccessorGenerator from shared objects file] [Loaded sun.reflect.MethodAccessorGenerator from shared objects file] [Loaded sun.reflect.ByteVectorFactory from shared objects file] [Loaded sun.reflect.ByteVector from shared objects file] [Loaded sun.reflect.ByteVectorImpl from shared objects file] [Loaded sun.reflect.ClassFileAssembler from shared objects file] [Loaded sun.reflect.UTF8 from shared objects file] [Loaded java.lang.Void from shared objects file] [Loaded sun.reflect.Label from shared objects file] [Loaded sun.reflect.Label$PatchInfo from shared objects file] [Loaded java.util.AbstractList$Itr from shared objects file] [Loaded sun.reflect.MethodAccessorGenerator$1 from shared objects file] [Loaded sun.reflect.ClassDefiner from shared objects file] [Loaded sun.reflect.ClassDefiner$1 from shared objects file] [Loaded sun.reflect.GeneratedMethodAccessor1 from __JVM_DefineClass__] Hello, 15
可以看到前15次反射調用A.foo()方法並沒有什麼稀奇的地方，但在第16次反射調用時似乎有什麼東西被觸發了，導致JVM新加載了一堆類，其中就包括[Loaded sun.reflect.GeneratedMethodAccessor1 from __JVM_DefineClass__]這麼一行。這是哪裏來的呢？
先來看看JDK裏Method.invoke()是怎麼實現的。
java.lang.reflect.Method：
publicfinalclass Method extends AccessibleObject implements GenericDeclaration, Member { // ...privatevolatile MethodAccessor methodAccessor; // For sharing of MethodAccessors. This branching structure is // currently only two levels deep (i.e., one root Method and // potentially many Method objects pointing to it.)private Method root; // ...public Object invoke(Object obj, Object... args) throws IllegalAccessException, IllegalArgumentException, InvocationTargetException { if (!override) { if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) { Class caller = Reflection.getCallerClass(1); Class targetClass = ((obj == null || !Modifier.isProtected(modifiers)) ? clazz : obj.getClass()); boolean cached; synchronized (this) { cached = (securityCheckCache == caller) && (securityCheckTargetClassCache == targetClass); } if (!cached) { Reflection.ensureMemberAccess(caller, clazz, obj, modifiers); synchronized (this) { securityCheckCache = caller; securityCheckTargetClassCache = targetClass; } } } } if (methodAccessor == null) acquireMethodAccessor(); return methodAccessor.invoke(obj, args); } // NOTE that there is no synchronization used here. It is correct // (though not efficient) to generate more than one MethodAccessor // for a given Method. However, avoiding synchronization will // probably make the implementation more scalable.privatevoid 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) { methodAccessor = tmp; return; } // Otherwise fabricate one and propagate it up to the root tmp = reflectionFactory.newMethodAccessor(this); setMethodAccessor(tmp); } // ... }
可以看到Method.invoke()實際上並不是自己實現的反射調用邏輯，而是委託給sun.reflect.MethodAccessor來處理。
每個實際的Java方法只有一個對應的Method對象作爲root，。這個root是不會暴露給用戶的，而是每次在通過反射獲取Method對象時新創建Method對象把root包裝起來再給用戶。在第一次調用一個實際Java方法對應得Method對象的invoke()方法之前，實現調用邏輯的MethodAccessor對象還沒創建；等第一次調用時才新創建MethodAccessor並更新給root，然後調用MethodAccessor.invoke()真正完成反射調用。
那麼MethodAccessor是啥呢？
sun.reflect.MethodAccessor：
publicinterface MethodAccessor { /** Matches specification in {@link java.lang.reflect.Method} */public Object invoke(Object obj, Object[] args) throws IllegalArgumentException, InvocationTargetException; }
可以看到它只是一個單方法接口，其invoke()方法與Method.invoke()的對應。
創建MethodAccessor實例的是ReflectionFactory。
sun.reflect.ReflectionFactory：
publicclass ReflectionFactory { privatestaticboolean initted = false; // ... //// "Inflation" mechanism. Loading bytecodes to implement // Method.invoke() and Constructor.newInstance() currently costs // 3-4x more than an invocation via native code for the first // invocation (though subsequent invocations have been benchmarked // to be over 20x faster). Unfortunately this cost increases // startup time for certain applications that use reflection // intensively (but only once per class) to bootstrap themselves. // To avoid this penalty we reuse the existing JVM entry points // for the first few invocations of Methods and Constructors and // then switch to the bytecode-based implementations. //// Package-private to be accessible to NativeMethodAccessorImpl // and NativeConstructorAccessorImplprivatestaticboolean noInflation = false; privatestaticint inflationThreshold = 15; // .../** We have to defer full initialization of this class until after the static initializer is run since java.lang.reflect.Method's static initializer (more properly, that for java.lang.reflect.AccessibleObject) causes this class's to be run, before the system properties are set up. */privatestaticvoid checkInitted() { if (initted) return; AccessController.doPrivileged(new PrivilegedAction() { public Object run() { // Tests to ensure the system properties table is fully // initialized. This is needed because reflection code is // called very early in the initialization process (before // command-line arguments have been parsed and therefore // these user-settable properties installed.) We assume that // if System.out is non-null then the System class has been // fully initialized and that the bulk of the startup code // has been run.if (System.out == null) { // java.lang.System not yet fully initializedreturnnull; } String val = System.getProperty("sun.reflect.noInflation"); if (val != null && val.equals("true")) { noInflation = true; } val = System.getProperty("sun.reflect.inflationThreshold"); if (val != null) { try { inflationThreshold = Integer.parseInt(val); } catch (NumberFormatException e) { throw (RuntimeException) new RuntimeException("Unable to parse property sun.reflect.inflationThreshold"). initCause(e); } } initted = true; returnnull; } }); } // ...public MethodAccessor newMethodAccessor(Method method) { checkInitted(); if (noInflation) { returnnew 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; } } }
這裏就可以看到有趣的地方了。如註釋所述，實際的MethodAccessor實現有兩個版本，一個是Java實現的，另一個是native code實現的。Java實現的版本在初始化時需要較多時間，但長久來說性能較好；native版本正好相反，啓動時相對較快，但運行時間長了之後速度就比不過Java版了。這是HotSpot的優化方式帶來的性能特性，同時也是許多虛擬機的共同點：跨越native邊界會對優化有阻礙作用，它就像個黑箱一樣讓虛擬機難以分析也將其內聯，於是運行時間長了之後反而是託管版本的代碼更快些。
爲了權衡兩個版本的性能，Sun的JDK使用了“inflation”的技巧：讓Java方法在被反射調用時，開頭若干次使用native版，等反射調用次數超過閾值時則生成一個專用的MethodAccessor實現類，生成其中的invoke()方法的字節碼，以後對該Java方法的反射調用就會使用Java版。
Sun的JDK是從1.4系開始採用這種優化的。
PS.可以在啓動命令里加上-Dsun.reflect.noInflation=true，就會RefactionFactory的noInflation屬性就變成true了，這樣不用等到15調用後，程序一開始就會用java版的MethodAccessor了。
上面看到了ReflectionFactory.newMethodAccessor()生產MethodAccessor的邏輯，在“開頭若干次”時用到的DelegatingMethodAccessorImpl代碼如下：
sun.reflect.DelegatingMethodAccessorImpl：
/** Delegates its invocation to another MethodAccessorImpl and can change its delegate at run time. */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; } }
這是一個間接層，方便在native與Java版的MethodAccessor之間實現切換。
然後下面就是native版MethodAccessor的Java一側的聲明:
sun.reflect.NativeMethodAccessorImpl：
/** Used only for the first few invocations of a Method; afterward, switches to bytecode-based implementation */class NativeMethodAccessorImpl extends MethodAccessorImpl { private Method method; private DelegatingMethodAccessorImpl parent; privateint numInvocations; NativeMethodAccessorImpl(Method method) { this.method = method; } public Object invoke(Object obj, Object[] args) throws IllegalArgumentException, InvocationTargetException { if (++numInvocations > ReflectionFactory.inflationThreshold()) { 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; } privatestaticnative Object invoke0(Method m, Object obj, Object[] args); }
每次NativeMethodAccessorImpl.invoke()方法被調用時，都會增加一個調用次數計數器，看超過閾值沒有；一旦超過，則調用MethodAccessorGenerator.generateMethod()來生成Java版的MethodAccessor的實現類，並且改變DelegatingMethodAccessorImpl所引用的MethodAccessor爲Java版。後續經由DelegatingMethodAccessorImpl.invoke()調用到的就是Java版的實現了。
注意到關鍵的invoke0()方法是個native方法。它在HotSpot VM裏是由JVM_InvokeMethod()函數所支持的：
由C編寫
JNIEXPORT jobject JNICALL Java_sun_reflect_NativeMethodAccessorImpl_invoke0 (JNIEnv *env, jclass unused, jobject m, jobject obj, jobjectArray args) { return JVM_InvokeMethod(env, m, obj, args); }
JVM_ENTRY(jobject, JVM_InvokeMethod(JNIEnv *env, jobject method, jobject obj, jobjectArray args0)) JVMWrapper("JVM_InvokeMethod"); Handle method_handle; if (thread->stack_available((address) &method_handle) >= JVMInvokeMethodSlack) { method_handle = Handle(THREAD, JNIHandles::resolve(method)); Handle receiver(THREAD, JNIHandles::resolve(obj)); objArrayHandle args(THREAD, objArrayOop(JNIHandles::resolve(args0))); oop result = Reflection::invoke_method(method_handle(), receiver, args, CHECK_NULL); jobject res = JNIHandles::make_local(env, result); if (JvmtiExport::should_post_vm_object_alloc()) { oop ret_type = java_lang_reflect_Method::return_type(method_handle()); assert(ret_type != NULL, "sanity check: ret_type oop must not be NULL!"); if (java_lang_Class::is_primitive(ret_type)) { // Only for primitive type vm allocates memory for java object. // See box() method. JvmtiExport::post_vm_object_alloc(JavaThread::current(), result); } } return res; } else { THROW_0(vmSymbols::java_lang_StackOverflowError()); } JVM_END
其中的關鍵又是Reflection::invoke_method()：
// This would be nicer if, say, java.lang.reflect.Method was a subclass // of java.lang.reflect.Constructor oop Reflection::invoke_method(oop method_mirror, Handle receiver, objArrayHandle args, TRAPS) { oop mirror = java_lang_reflect_Method::clazz(method_mirror); int slot = java_lang_reflect_Method::slot(method_mirror); booloverride = java_lang_reflect_Method::override(method_mirror) != 0; objArrayHandle ptypes(THREAD, objArrayOop(java_lang_reflect_Method::parameter_types(method_mirror))); oop return_type_mirror = java_lang_reflect_Method::return_type(method_mirror); BasicType rtype; if (java_lang_Class::is_primitive(return_type_mirror)) { rtype = basic_type_mirror_to_basic_type(return_type_mirror, CHECK_NULL); } else { rtype = T_OBJECT; } instanceKlassHandle klass(THREAD, java_lang_Class::as_klassOop(mirror)); methodOop m = klass->method_with_idnum(slot); if (m == NULL) { THROW_MSG_0(vmSymbols::java_lang_InternalError(), "invoke"); } methodHandle method(THREAD, m); return invoke(klass, method, receiver, override, ptypes, rtype, args, true, THREAD); }
再下去就深入到HotSpot VM的內部了，本文就在這裏打住吧。有同學有興趣深究的話以後可以再寫一篇討論native版的實現。
回到Java的一側。MethodAccessorGenerator長啥樣呢？由於代碼太長，這裏就不完整貼了，有興趣的可以到OpenJDK 6的Mercurial倉庫看：OpenJDK 6 build 17的MethodAccessorGenerator。它的基本工作就是在內存裏生成新的專用Java類，並將其加載。就貼這麼一個方法：
privatestaticsynchronized String generateName(boolean isConstructor, boolean forSerialization) { if (isConstructor) { if (forSerialization) { int num = ++serializationConstructorSymnum; return "sun/reflect/GeneratedSerializationConstructorAccessor" + num; } else { int num = ++constructorSymnum; return "sun/reflect/GeneratedConstructorAccessor" + num; } } else { int num = ++methodSymnum; return "sun/reflect/GeneratedMethodAccessor" + num; } }
去閱讀源碼的話，可以看到MethodAccessorGenerator是如何一點點把Java版的MethodAccessor實現類生產出來的。也可以看到GeneratedMethodAccessor+數字這種名字是從哪裏來的了，就在上面的generateName()方法裏。
對本文開頭的例子的A.foo()，生成的Java版MethodAccessor大致如下：
package sun.reflect; publicclass GeneratedMethodAccessor1 extends MethodAccessorImpl { public GeneratedMethodAccessor1() { super(); } public Object invoke(Object obj, Object[] args) throws IllegalArgumentException, InvocationTargetException { // prepare the target and parametersif (obj == null) thrownew NullPointerException(); try { A target = (A) obj; if (args.length != 1) thrownew IllegalArgumentException(); String arg0 = (String) args[0]; } catch (ClassCastException e) { thrownew IllegalArgumentException(e.toString()); } catch (NullPointerException e) { thrownew IllegalArgumentException(e.toString()); } // make the invocationtry { target.foo(arg0); } catch (Throwable t) { thrownew InvocationTargetException(t); } } }
就反射調用而言，這個invoke()方法非常乾淨（然而就“正常調用”而言這額外開銷還是明顯的）。注意到參數數組被拆開了，把每個參數都恢復到原本沒有被Object[]包裝前的樣子，然後對目標方法做正常的invokevirtual調用。由於在生成代碼時已經循環遍歷過參數類型的數組，生成出來的代碼裏就不再包含循環了。
至此找到我的答案了，因爲MethodAccessor會做強制類型轉換再進行方法調用，但父類強制轉化成子類的的時候就會報錯類型不匹配錯誤了，所以如果變量的引用聲明是父但實際指向的對象是子，那麼這種調用也是可以的。
----------------------------------------------------------題外話------------------------------------------
當該反射調用成爲熱點時，它甚至可以被內聯到靠近Method.invoke()的一側，大大降低了反射調用的開銷。而native版的反射調用則無法被有效內聯，因而調用開銷無法隨程序的運行而降低。
雖說Sun的JDK這種實現方式使得反射調用方法成本比以前降低了很多，但Method.invoke()本身要用數組包裝參數；而且每次調用都必須檢查方法的可見性（在Method.invoke()裏），也必須檢查每個實際參數與形式參數的類型匹配性（在NativeMethodAccessorImpl.invoke0()裏或者生成的Java版MethodAccessor.invoke()裏）；而且Method.invoke()就像是個獨木橋一樣，各處的反射調用都要擠過去，在調用點上收集到的類型信息就會很亂，影響內聯程序的判斷，使得Method.invoke()自身難以被內聯到調用方。
相比之下JDK7裏新的MethodHandler則更有潛力，在其功能完全實現後能達到比普通反射調用方法更高的性能。在使用MethodHandle來做反射調用時，MethodHandle.invoke()的形式參數與返回值類型都是準確的，所以只需要在鏈接方法的時候才需要檢查類型的匹配性，而不必在每次調用時都檢查。而且MethodHandle是不可變值，在創建後其內部狀態就不會再改變了；JVM可以利用這個知識而放心的對它做激進優化，例如將實際的調用目標內聯到做反射調用的一側。
本來Java的安全機制使得不同類之間不是任意信息都可見，但Sun的JDK裏開了個口，有一個標記類專門用於開後門：
package sun.reflect; /** <P> MagicAccessorImpl (named for parity with FieldAccessorImpl and others, not because it actually implements an interface) is a marker class in the hierarchy. All subclasses of this class are "magically" granted access by the VM to otherwise inaccessible fields and methods of other classes. It is used to hold the code for dynamically-generated FieldAccessorImpl and MethodAccessorImpl subclasses. (Use of the word "unsafe" was avoided in this class's name to avoid confusion with {@link sun.misc.Unsafe}.) </P> <P> The bug fix for 4486457 also necessitated disabling verification for this class and all subclasses, as opposed to just SerializationConstructorAccessorImpl and subclasses, to avoid having to indicate to the VM which of these dynamically-generated stub classes were known to be able to pass the verifier. </P> <P> Do not change the name of this class without also changing the VM's code. </P> */class MagicAccessorImpl { }
那個"__JVM_DefineClass__"的來源是這裏：
src/share/vm/prims/jvm.cpp
// common code for JVM_DefineClass() and JVM_DefineClassWithSource() // and JVM_DefineClassWithSourceCond()static jclass jvm_define_class_common(JNIEnv *env, constchar *name, jobject loader, const jbyte *buf, jsize len, jobject pd, constchar *source, jboolean verify, TRAPS) { if (source == NULL) source = "__JVM_DefineClass__";
P.S. log裏的"shared objects file",其實就是rt.jar,爲什麼要這麼顯示，Stack OverFlow上有這樣的回答：
This is Class Data Sharing. When running the Sun/Oracle Client HotSpot and sharing enable (either -Xshare:auto which
is the default, or -Xshare:on), the classes.jsa file
is memory mapped. This file contains a number of classes (listed in the classlist file) in internal
representation suitable for the exact configuration of the machine running it. The idea is that the classes can be loaded quickly, getting the the JVM up faster. Soon enough a class not covered will be hit, and rt.jar will
need to be opened and classes loaded conventionally as required.
不能很好理解，大概理解就是所有jvm共享，並可以快速加載裏面的class.有英文好的朋友可以留言幫助下。
P.S java內聯函數
C++是否爲內聯函數由自己決定，Java由編譯器決定。內聯函數就是指函數在被調用的地方直接展開，編譯器在調用時不用像一般函數那樣，參數壓棧，返回時參數出棧以及資源釋放等，這樣提高了程序執行速度。
Java不支持直接聲明爲內聯函數的，如果想讓他內聯，則是由編譯器說了算，你只能夠向編譯器提出請求。
final除了不能被override外，還可能實現內聯。如果函數爲private，則也可能是內聯的。
總的來說，一般的函數都不會被當做內聯函數，只有聲明瞭final後，編譯器纔會考慮是不是要把你的函數變成內聯函數。
內聯不一定好，當被指定爲內聯的方法體很大時，展開的開銷可能就已經超過了普通函數調用調用的時間，引入了內聯反而降低了性能，因爲在選擇這個關鍵字應該慎重些，不過，在以後高版本的JVM中，在處理內聯時做出了優化，它會根據方法的規模來確定是否展開調用。