JVM反射原理是什么-创新互联
这篇文章主要介绍了JVM反射原理是什么,具有一定借鉴价值,需要的朋友可以参考下。希望大家阅读完这篇文章后大有收获。下面让小编带着大家一起了解一下。
成都创新互联网站建设由有经验的网站设计师、开发人员和项目经理组成的专业建站团队,负责网站视觉设计、用户体验优化、交互设计和前端开发等方面的工作,以确保网站外观精美、网站建设、成都做网站易于使用并且具有良好的响应性。反射定义
1,JAVA反射机制是在运行状态中
对于任意一个类,都能够知道这个类的所有属性和方法;
对于任意一个对象,都能够调用它的任意一个方法和属性;
这种动态获取的信息以及动态调用对象的方法的功能称为java语言的反射机制。
反射提供的功能:
- 在运行时判断任意一个对象所属的类
- 在运行时构造任意一个类的对象
- 在运行时判断任意一个类所具有的成员变量和方法
- 在运行时调用任意一个对象的方法
(如果属性是private,正常情况下是不允许外界操作属性值,这里可以用Field类的setAccessible(true)方法,暂时打开操作的权限)
反射的使用场景
- Java编码时知道类和对象的具体信息,此时直接对类和对象进行操作即可,无需反射
- 如果编码时不知道类或者对象的具体信息,此时应该使用反射来实现
反射源码解析
举例API :
Class.forName("com.my.reflectTest").newInstance()复制代码
1. 反射获取类实例 Class.forName("xxx");
首先调用了 java.lang.Class 的静态方法,获取类信息!
注意: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 入参ClassLoader 进类加载!
@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); }复制代码
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); }复制代码
3. newInstance()
newInstance() 其实相当于调用类的无参构造函数,主要做了三件事复制代码
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;
// 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 Constructorc = 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 () { public Void run() { c.setAccessible(true); return null; } }); cachedConstructor = c; } catch (NoSuchMethodException e) { throw (InstantiationException) new InstantiationException(getName()).initCause(e); } } Constructor 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; } }复制代码
4. getConstructor0() 为获取匹配的构造方器;分三步:
1. 先获取所有的constructors, 然后通过进行参数类型比较; 2. 找到匹配后,通过 ReflectionFactory copy一份constructor返回; 3. 否则抛出 NoSuchMethodException;
private ConstructorgetConstructor0(Class>[] parameterTypes, int which) throws NoSuchMethodException { // 获取所有构造器 Constructor [] constructors = privateGetDeclaredConstructors((which == Member.PUBLIC)); for (Constructor constructor : constructors) { if (arrayContentsEq(parameterTypes, constructor.getParameterTypes())) { return getReflectionFactory().copyConstructor(constructor); } } throw new NoSuchMethodException(getName() + ". " + argumentTypesToString(parameterTypes)); }复制代码
5. privateGetDeclaredConstructors(), 获取所有的构造器主要步骤;
1. 先尝试从缓存中获取; 2. 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;
// 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[] privateGetDeclaredConstructors(boolean publicOnly) { checkInitted(); Constructor [] 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 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 [] temporaryRes = (Constructor []) 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 reflectionData() { SoftReference > reflectionData = this.reflectionData; int classRedefinedCount = this.classRedefinedCount; ReflectionData 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 newReflectionData(SoftReference > 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 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; } } }复制代码
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{ volatile Field[] declaredFields; volatile Field[] publicFields; volatile Method[] declaredMethods; volatile Method[] publicMethods; volatile Constructor [] declaredConstructors; volatile Constructor [] 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; } }复制代码
6.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); }复制代码
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。
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文章标题:JVM反射原理是什么-创新互联
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