Reflection definition
1. JAVA reflection mechanism is in the running state
For any class, all attributes and methods of this class can be known;
For any object, any of its methods and properties can be called;
The function of dynamically acquiring information and dynamically calling methods of objects is called the reflection mechanism of java language.
Functions provided by reflection:
- Determine the class of any object at runtime
- Construct an object of any class at runtime
- Judge the member variables and methods of any class at runtime
- Call any object's method at runtime
(If the attribute is private, the outside world is not allowed to manipulate the attribute value under normal circumstances, here you can use the setAccessible(true) method of the Field class to temporarily open the operation authority)
Use scenarios for reflection
- Know the specific information of the class and object when coding in Java. At this time, you can directly operate on the class and object without reflection.
- If you do not know the specific information of the class or object when coding, you should use reflection to achieve
Reflection source code analysis
Example API:
Class.forName("com.my.reflectTest").newInstance()1. Reflect to obtain class instance Class.forName("xxx");
First call the static method of java.lang.Class to get the class information!
Note: forName() to obtain class information by reflection does not leave the implementation to java, but to jvm to load it!
The main thing is to get the ClassLoader first, then call the native method to get the information, and the loading class is to call back into the parameter ClassLoader to load the class!
@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);
}2. java.lang.ClassLoader-----loadClass()
// 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);
}3. newInstance()
newInstance() 其实相当于调用类的无参构造函数,主要做了三件事
-
Permission detection, if not passed, throw an exception directly;
-
Find the parameterless constructor and cache it;
- Call the parameterless construction method of the specific method, generate an instance and return;
// 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;
}
}4. getConstructor0() is to obtain the matching constructor; there are three steps:
1. Get all the constructors first, and then compare the parameter types;
2. After finding a match, copy a copy of the constructor through ReflectionFactory to return;
3. Otherwise, throw NoSuchMethodException;
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));
}5. privateGetDeclaredConstructors(), the main steps to get all the constructors;
1. Try to get it from the cache first;
2. If the cache is not available, get it from the jvm again and store it in the cache. The cache is saved with a soft reference to ensure that the memory is available;
// 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 cla***edefinedCount = this.cla***edefinedCount;
ReflectionData<T> rd;
if (useCaches &&
reflectionData != null &&
(rd = reflectionData.get()) != null &&
rd.redefinedCount == cla***edefinedCount) {
return rd;
}
// else no SoftReference or cleared SoftReference or stale ReflectionData
// -> create and replace new instance
return newReflectionData(reflectionData, cla***edefinedCount);
}
// 新创建缓存，保存反射信息
private ReflectionData<T> newReflectionData(SoftReference<ReflectionData<T>> oldReflectionData,
int cla***edefinedCount) {
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<>(cla***edefinedCount);
// 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;
cla***edefinedCount = this.cla***edefinedCount;
if (oldReflectionData != null &&
(rd = oldReflectionData.get()) != null &&
rd.redefinedCount == cla***edefinedCount) {
return rd;
}
}
}In addition, use relactionData() for cache storage; the data structure of ReflectionData is as follows!
// 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 cla***edefinedCount when we created this ReflectionData instance
final int redefinedCount;
ReflectionData(int redefinedCount) {
this.redefinedCount = redefinedCount;
}
}6. Through the above, the Constructor is obtained! Then just call the newInstance() of its corresponding constructor to return the instance!
// 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);
}After returning the instance of the constructor, you can perform type conversion according to the outside, so that you can use the interface or method to call the instance function.
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