关于后端:建议收藏超详细的JVM反射原理技术点总结

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反射定义

1,JAVA 反射机制是在 运行状态中

对于任意一个类,都可能晓得这个类的所有属性和办法;

对于任意一个对象,都可能调用它的任意一个办法和属性;

这种动静获取的信息以及动静调用对象的办法的性能称为 java 语言的反射机制。

反射提供的性能:

  • 在运行时判断任意一个对象所属的类
  • 在运行时结构任意一个类的对象
  • 在运行时判断任意一个类所具备的成员变量和办法
  • 在运行时调用任意一个对象的办法

    (如果属性是 private,失常状况下是不容许外界操作属性值,这里能够用 Field 类的 setAccessible(true)办法,临时关上操作的权限)

反射的应用场景

  • Java 编码时晓得类和对象的具体信息,此时间接对类和对象进行操作即可,无需反射
  • 如果编码时不晓得类或者对象的具体信息,此时应该应用反射来实现

反射源码解析

举例 API:

Class.forName("com.my.reflectTest").newInstance()

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!

留神: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!

          次要是先获取 ClassLoader, 而后调用 native 办法,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,从而获取以后的 classLoader
        Class<?> caller = Reflection.getCallerClass();
        // 调用 native 办法进行获取 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
            // 如果曾经加载了的话,就不必再加载了
            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,再本人加载
                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) {
            // 应用 ConcurrentHashMap 来保留锁
            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, 次要做了三件事
  • 权限检测,如果不通过间接抛出异样;
  • 查找无参结构器,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;
// 首先必定是 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,首先要找到其无参结构器
        if (cachedConstructor == null) {if (this == Class.class) {// 不容许调用 Class 的 newInstance() 办法
                throw new IllegalAccessException("Can not call newInstance() on the Class for java.lang.Class"
                );
            }
            try {
                // 获取无参结构器
                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 {
            // 调用无参结构器
            return tmpConstructor.newInstance((Object[])null);
        } catch (InvocationTargetException e) {Unsafe.getUnsafe().throwException(e.getTargetException());
            // Not reached
            return null;
        }
    }

4. getConstructor0() 为获取匹配的结构方器;分三步:

1. 先获取所有的 constructors, 而后通过进行参数类型比拟;
2. 找到匹配后,通过 ReflectionFactory copy 一份 constructor 返回;
3. 否则抛出 NoSuchMethodException;

private Constructor<T> getConstructor0(Class<?>[] parameterTypes,
                                        int which) throws NoSuchMethodException
    {
        // 获取所有结构器
        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(), 获取所有的结构器次要步骤;

1. 先尝试从缓存中获取;
2. 如果缓存没有,则从 jvm 中从新获取,并存入缓存,缓存应用软援用进行保留,保障内存可用;

// 获取以后类所有的构造方法,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(), 获取保留的信息,应用软援用保留,从而使内存不够能够回收
        ReflectionData<T> rd = reflectionData();
        if (rd != null) {
            res = publicOnly ? rd.publicConstructors : rd.declaredConstructors;
            // 存在缓存,则间接返回
            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 {
            // 应用 native 办法从 jvm 获取结构器
            res = getDeclaredConstructors0(publicOnly);
        }
        if (rd != null) {
            // 最初,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 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);
    }
    
    // 新创建缓存,保留反射信息
    private ReflectionData<T> newReflectionData(SoftReference<ReflectionData<T>> oldReflectionData,
                                                int classRedefinedCount) {if (!useCaches) return null;
 
        // 应用 cas 保障更新的线程安全性,所以反射是保障线程平安的
        while (true) {ReflectionData<T> rd = new ReflectionData<>(classRedefinedCount);
            // try to CAS it...
            if (Atomic.casReflectionData(this, oldReflectionData, new SoftReference<>(rd))) {return rd;}
            // 先应用 CAS 更新,如果更新胜利,则立刻返回,否则测查以后已被其余线程更新的状况,如果和本人想要更新的状态统一,则也算是胜利了
            oldReflectionData = this.reflectionData;
            classRedefinedCount = this.classRedefinedCount;
            if (oldReflectionData != null &&
                (rd = oldReflectionData.get()) != null &&
                rd.redefinedCount == classRedefinedCount) {return rd;}
        }
    }

另外,应用 relactionData() 进行缓存保留;ReflectionData 的数据结构如下!

// 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;}
    }

6. 通过下面,获取到 Constructor 了!接下来就只需调用其相应结构器的 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 办法,进行调用 constructor
        return newInstance0(c, args);
    }

返回结构器的实例后,能够依据内部进行进行类型转换,从而应用接口或办法进行调用实例性能了。

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