[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次反射调用A.foo()方法并没有什么稀奇的地方，但在第16次反射调用时似乎有什么东西被触发了，导致JVM新加载了一堆类，其中就包括[Loaded sun.reflect.GeneratedMethodAccessor1 from __JVM_DefineClass__]这么一行。这是哪里来的呢？
先来看看JDK里Method.invoke()是怎么实现的。
java.lang.reflect.Method：
public final
class Method extends AccessibleObject implements GenericDeclaration,
Member {
// ...
private volatile 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.
private void 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);
}
// ...
}
public interface MethodAccessor {
/** Matches specification in {@link java.lang.reflect.Method} */
public Object invoke(Object obj, Object[] args)
throws IllegalArgumentException, InvocationTargetException;
}
可以看到它只是一个单方法接口，其invoke()方法与Method.invoke()的对应。
创建MethodAccessor实例的是ReflectionFactory。
sun.reflect.ReflectionFactory：
public class ReflectionFactory {
private static boolean 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 NativeConstructorAccessorImpl
private static boolean noInflation = false;
private static int 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. */
private static void 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 initialized
return null;
}
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;
return null;
}
});
}
// ...
public MethodAccessor newMethodAccessor(Method method) {
checkInitted();
if (noInflation) {
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;
}
}
}
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 {
}
那个"__JVM_DefineClass__"的来源是这里：
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, const char *name,
jobject loader, const jbyte *buf,
jsize len, jobject pd, const char *source,
jboolean verify, TRAPS) {
if (source == NULL) source = "__JVM_DefineClass__";
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.
[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次反射调用A.foo()方法并没有什么稀奇的地方，但在第16次反射调用时似乎有什么东西被触发了，导致JVM新加载了一堆类，其中就包括[Loaded sun.reflect.GeneratedMethodAccessor1 from __JVM_DefineClass__]这么一行。这是哪里来的呢？
先来看看JDK里Method.invoke()是怎么实现的。
java.lang.reflect.Method：
public final
class Method extends AccessibleObject implements GenericDeclaration,
Member {
// ...
private volatile 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.
private void 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);
}
// ...
}
public interface MethodAccessor {
/** Matches specification in {@link java.lang.reflect.Method} */
public Object invoke(Object obj, Object[] args)
throws IllegalArgumentException, InvocationTargetException;
}
可以看到它只是一个单方法接口，其invoke()方法与Method.invoke()的对应。
创建MethodAccessor实例的是ReflectionFactory。
sun.reflect.ReflectionFactory：
public class ReflectionFactory {
private static boolean 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 NativeConstructorAccessorImpl
private static boolean noInflation = false;
private static int 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. */
private static void 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 initialized
return null;
}
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;
return null;
}
});
}
// ...
public MethodAccessor newMethodAccessor(Method method) {
checkInitted();
if (noInflation) {
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;
}
}
}
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 {
}
那个"__JVM_DefineClass__"的来源是这里：
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, const char *name,
jobject loader, const jbyte *buf,
jsize len, jobject pd, const char *source,
jboolean verify, TRAPS) {
if (source == NULL) source = "__JVM_DefineClass__";
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.
