java基礎(chǔ)欄目今天介紹超詳細(xì)的JVM反射原理技術(shù)點(diǎn)總結(jié)哦。

反射定義

1，JAVA反射機(jī)制是在運(yùn)行狀態(tài)中

對(duì)于任意一個(gè)類，都能夠知道這個(gè)類的所有屬性和方法；

對(duì)于任意一個(gè)對(duì)象，都能夠調(diào)用它的任意一個(gè)方法和屬性；

這種動(dòng)態(tài)獲取的信息以及動(dòng)態(tài)調(diào)用對(duì)象的方法的功能稱為java語(yǔ)言的反射機(jī)制。

反射提供的功能：

• 在運(yùn)行時(shí)判斷任意一個(gè)對(duì)象所屬的類
• 在運(yùn)行時(shí)構(gòu)造任意一個(gè)類的對(duì)象
• 在運(yùn)行時(shí)判斷任意一個(gè)類所具有的成員變量和方法
• 在運(yùn)行時(shí)調(diào)用任意一個(gè)對(duì)象的方法

? ??（如果屬性是private，正常情況下是不允許外界操作屬性值，這里可以用Field類的setAccessible(true)方法，暫時(shí)打開(kāi)操作的權(quán)限）

反射的使用場(chǎng)景

• Java編碼時(shí)知道類和對(duì)象的具體信息，此時(shí)直接對(duì)類和對(duì)象進(jìn)行操作即可，無(wú)需反射
• 如果編碼時(shí)不知道類或者對(duì)象的具體信息，此時(shí)應(yīng)該使用反射來(lái)實(shí)現(xiàn)

反射源碼解析

舉例API ：?

Class.forName("com.my.reflectTest").newInstance()復(fù)制代碼

1. 反射獲取類實(shí)例 Class.forName("xxx");

　　首先調(diào)用了 java.lang.Class 的靜態(tài)方法，獲取類信息！

注意：forName()反射獲取類信息，并沒(méi)有將實(shí)現(xiàn)留給了java,而是交給了jvm去加載！

主要是先獲取 ClassLoader, 然后調(diào)用 native 方法，獲取信息，加載類則是回調(diào) 入?yún)lassLoader 進(jìn)類加載！

 @CallerSensitive
    public static Class<?> forName(String className)
                throws ClassNotFoundException {
        // 先通過(guò)反射，獲取調(diào)用進(jìn)來(lái)的類信息，從而獲取當(dāng)前的 classLoader
        Class<?> caller = Reflection.getCallerClass();
        // 調(diào)用native方法進(jìn)行獲取class信息
        return forName0(className, true, ClassLoader.getClassLoader(caller), caller);
    }復(fù)制代碼

2. java.lang.ClassLoader-----loadClass()

// java.lang.ClassLoader
    protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        // 先獲取鎖
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            // 如果已經(jīng)加載了的話，就不用再加載了
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    // 雙親委托加載
                    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
                }
 
                // 父類沒(méi)有加載到時(shí)，再自己加載
                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來(lái)保存鎖
            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);
    }復(fù)制代碼

3. newInstance()

newInstance() 其實(shí)相當(dāng)于調(diào)用類的無(wú)參構(gòu)造函數(shù),主要做了三件事復(fù)制代碼

• 權(quán)限檢測(cè)，如果不通過(guò)直接拋出異常；

• 查找無(wú)參構(gòu)造器，并將其緩存起來(lái)；

• 調(diào)用具體方法的無(wú)參構(gòu)造方法，生成實(shí)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() 其實(shí)相當(dāng)于調(diào)用類的無(wú)參構(gòu)造函數(shù)，所以，首先要找到其無(wú)參構(gòu)造器
        if (cachedConstructor == null) {
            if (this == Class.class) {
                // 不允許調(diào)用 Class 的 newInstance() 方法
                throw new IllegalAccessException(
                    "Can not call newInstance() on the Class for java.lang.Class"
                );
            }
            try {
                // 獲取無(wú)參構(gòu)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&#39;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 {
            // 調(diào)用無(wú)參構(gòu)造器
            return tmpConstructor.newInstance((Object[])null);
        } catch (InvocationTargetException e) {
            Unsafe.getUnsafe().throwException(e.getTargetException());
            // Not reached
            return null;
        }
    }復(fù)制代碼

4. getConstructor0() 為獲取匹配的構(gòu)造方器；分三步：

　　1. 先獲取所有的constructors, 然后通過(guò)進(jìn)行參數(shù)類型比較； 　　2. 找到匹配后，通過(guò) ReflectionFactory copy一份constructor返回； 　　3. 否則拋出 NoSuchMethodException;

private Constructor<T> getConstructor0(Class<?>[] parameterTypes,
                                        int which) throws NoSuchMethodException
    {
        // 獲取所有構(gòu)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));
    }復(fù)制代碼

5. privateGetDeclaredConstructors(), 獲取所有的構(gòu)造器主要步驟；

　　1. 先嘗試從緩存中獲?。?　　2. 如果緩存沒(méi)有，則從jvm中重新獲取，并存入緩存，緩存使用軟引用進(jìn)行保存，保證內(nèi)存可用；

// 獲取當(dāng)前類所有的構(gòu)造方法，通過(guò)jvm或者緩存
    // 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;
        // 調(diào)用 reflectionData(), 獲取保存的信息，使用軟引用保存，從而使內(nèi)存不夠可以回收
        ReflectionData<T> rd = reflectionData();
        if (rd != null) {
            res = publicOnly ? rd.publicConstructors : rd.declaredConstructors;
            // 存在緩存，則直接返回
            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方法從jvm獲取構(gòu)造器
            res = getDeclaredConstructors0(publicOnly);
        }
        if (rd != null) {
            // 最后，將從jvm中讀取的內(nèi)容，存入緩存
            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);
    }
    
    // 新創(chuàng)建緩存，保存反射信息
    private ReflectionData<T> newReflectionData(SoftReference<ReflectionData<T>> oldReflectionData,
                                                int classRedefinedCount) {
        if (!useCaches) return null;
 
        // 使用cas保證更新的線程安全性，所以反射是保證線程安全的
        while (true) {
            ReflectionData<T> 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，則立即返回，否則測(cè)查當(dāng)前已被其他線程更新的情況，如果和自己想要更新的狀態(tài)一致，則也算是成功了
            oldReflectionData = this.reflectionData;
            classRedefinedCount = this.classRedefinedCount;
            if (oldReflectionData != null &&
                (rd = oldReflectionData.get()) != null &&
                rd.redefinedCount == classRedefinedCount) {
                return rd;
            }
        }
    }復(fù)制代碼

另外，使用 relactionData() 進(jìn)行緩存保存；ReflectionData 的數(shù)據(jù)結(jié)構(gòu)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;
        }
    }復(fù)制代碼

6.通過(guò)上面，獲取到 Constructor 了！接下來(lái)就只需調(diào)用其相應(yīng)構(gòu)造器的 newInstance()，即返回實(shí)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&#39;t inflate a constructor belonging to a vm-anonymous class
        // because that kind of class can&#39;t be referred to by name, hence can&#39;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);
        }
 
        // 調(diào)用native方法，進(jìn)行調(diào)用 constructor
        return newInstance0(c, args);
    }復(fù)制代碼

返回構(gòu)造器的實(shí)例后，可以根據(jù)外部進(jìn)行進(jìn)行類型轉(zhuǎn)換，從而使用接口或方法進(jìn)行調(diào)用實(shí)例功能了。

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