2023面试笔记四

1、gc导致的cpu冲高

排查是否为gc导致，看如下两点：

• gc频率和耗时

• 内存占用率

（1）gc频率和耗时有两种手段看：

第一种：根据gc日志的打印时间，可确定每次gc间隔的时间和耗时：

使用这种方式的前提是配置了gc日志的打印，参考jvm参数如下：

-XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintHeapAtGC -Xloggc:../../logs/gc.log

第二种：使用如下命令，可每隔5s打印一次gc情况，从打印结果可以判断gc频率，在通过YGCT/YGC和FGCT/FGC可以计算出每次gc的耗时。

jstat -gc  5000

Jstat是JDK自带的一个轻量级小工具。它位于java的bin目录下，主要利用JVM内建的指令对Java应用程序的资源和性能进行实时的命令行的监控，包括了对Heap size和垃圾回收状况的监控。

命令用法：jstat [-命令选项] [pid] [间隔时间/毫秒] [查询次数]

若gc频率低，且耗时短，则基本可以排除是gc导致的cpu冲高问题。

• S0C：年轻代中 To Survivor 的容量（单位 KB）；
• S1C：年轻代中 From Survivor 的容量（单位 KB）；
• S0U：年轻代中 To Survivor 目前已使用空间（单位 KB）；
• S1U：年轻代中 From Survivor 目前已使用空间（单位 KB）；
• EC：年轻代中 Eden 的容量（单位 KB）；
• EU：年轻代中 Eden 目前已使用空间（单位 KB）；
• OC：老年代的容量（单位 KB）；
• OU：老年代目前已使用空间（单位 KB）；
• MC：Metaspace 的容量（单位 KB）；
• MU：Metaspace 目前已使用空间（单位 KB）；
• CCSC：压缩类空间大小
• CCSU：压缩类空间使用大小
• YGC：从应用程序启动到采样时年轻代中 gc 次数；
• YGCT：从应用程序启动到采样时年轻代中 gc 所用时间 (s)；
• FGC：从应用程序启动到采样时 old 代（全 gc）gc 次数；
• FGCT：从应用程序启动到采样时 old 代（全 gc）gc 所用时间 (s)；
• GCT：从应用程序启动到采样时 gc 用的总时间 (s)

（2）jvm内存占用

使用jmap命令导出dump文件

jmap -dump:format=b,file=/tmp/1/dump.hprof 3618

也可以通过arthas工具，导出dump文件

heapdump /tmp/1/dump.hprof

通过堆栈信息分析占用内存的对象是哪些代码。

2、aqs

AbstractQueuedSynchronizer 抽象队列同步器。

抢到资源的线程直接使用处理业务逻辑，抢不到资源的必然涉及一种排队等候机制。抢占资源失败的线程继续去等待(类似银行业务办理窗口都满了，暂时没有受理窗口的顾客只能去候客区排队等候)，但等候线程仍然保留获取锁的可能且获取锁流程仍在继续(候客区的顾客也在等着叫号，轮到了再去受理窗口办理业务)。

既然说到了排队等候机制，那么就一定会有某种队列形成，这样的队列是什么数据结构呢?

如果共享资源被占用，就需要一定的阻塞等待唤醒机制来保证锁分配。这个机制主要用的是CLH队列的变体实现的，将暂时获取不到锁的线程加入到队列中，这个队列就是AQS的抽象表现。它将请求共享资源的线程封装成队列的结点(Node)，通过CAS、自旋以及LockSupportpark)的方式，维护state变量的状态，使并发达到同步的控制效果。
总结：state变量+CLH变种的双端队列(FIFO)

2.1、aqs是什么

AQS同步器是Java并发编程的基础，从资源共享的角度分成独占和共享两种模式，像ReentrantLock、ThreadPoolExecutor、CountDownLatch等都是基于AQS来实现的，如图：

2.2、AQS同步队列的基本结构

AQS维护了一个头节点（head）和一个尾节点（tail）结构的双向链表，当一个线程获取锁失败时，会将该线程打包成一个Node节点，挂到同步队列尾节点

private transient volatile Node head;//同步队列头结点
private transient volatile Node tail;//同步队列尾结点
private volatile int state;//同步状态 

2.3、从ReentrantLock开始

 * A reentrant mutual exclusion {@link Lock} with the same basic
public class ReentrantLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = 7373984872572414699L;
    /** Synchronizer providing all implementation mechanics */
    private final Sync sync;

     * Base of synchronization control for this lock. Subclassed
    abstract static class Sync extends AbstractQueuedSynchronizer {

     * Sync object for non-fair locks
    static final class NonfairSync extends Sync {

     * Sync object for fair locks
    static final class FairSync extends Sync {

     * Creates an instance of {@code ReentrantLock}.
    public ReentrantLock() {
        sync = new NonfairSync();
    }

     * Creates an instance of {@code ReentrantLock} with the
    public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }

     * Acquires the lock.
    public void lock() {
        sync.lock();//<------------------------注意，我们从这里入手
    }
        
    * Attempts to release this lock.
    public void unlock() {
        sync.release(1);
    }
    ...
}

从最简单的lock方法开始看看公平和非公平，先浏览下AbstractQueuedSynchronizer，FairSync，NonfairSync类的源码。

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {

	...

     * Acquires in exclusive mode, ignoring interrupts.  Implemented
    public final void acquire(int arg) {//公平锁或非公平锁都会调用这方法
        if (!tryAcquire(arg) &&//0.
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))//1. 2.
            selfInterrupt();//3.
    }
    
    //0.
    * Attempts to acquire in exclusive mode. This method should query
    protected boolean tryAcquire(int arg) {//取决于公平锁或非公平锁的实现
        throw new UnsupportedOperationException();
    }
	
    
    //1.
    * Acquires in exclusive uninterruptible mode for thread already in
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
    
    //2.
    * Creates and enqueues node for current thread and given mode.
    private Node addWaiter(Node mode) {
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail;
        if (pred != null) {
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }
    
    //3.
    static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }
    
    //这个方法将会被公平锁的tryAcquire()调用
    * Queries whether any threads have been waiting to acquire longer
    public final boolean hasQueuedPredecessors() {
        // The correctness of this depends on head being initialized
        // before tail and on head.next being accurate if the current
        // thread is first in queue.
        Node t = tail; // Read fields in reverse initialization order
        Node h = head;
        Node s;
        return h != t &&
            ((s = h.next) == null || s.thread != Thread.currentThread());
    }
    
    
	...         
}

public class ReentrantLock implements Lock, java.io.Serializable {
    
    ...
    
    //非公平锁与公平锁的公共父类
     * Base of synchronization control for this lock. Subclassed
    abstract static class Sync extends AbstractQueuedSynchronizer {
    
    	...
        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        } 
        ...
    
    }
        
    //非公平锁
	static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {//<---ReentrantLock初始化为非公平锁时，ReentrantLock.lock()将会调用这
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);//调用父类AbstractQueuedSynchronizer的acquire()
        }

        //acquire()将会间接调用该方法
        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);//调用父类Sync的nonfairTryAcquire()
        }
    }
    
    * Sync object for fair locks
    static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;

        final void lock() {//<---ReentrantLock初始化为非公平锁时，ReentrantLock.lock()将会调用这
            acquire(1);调用父类AbstractQueuedSynchronizer的acquire()
        }

        //acquire()将会间接调用该方法
         * Fair version of tryAcquire.  Don't grant access unless
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!hasQueuedPredecessors() &&//<---公平锁与非公平锁的唯一区别，公平锁调用hasQueuedPredecessors()，而非公平锁没有调用
                    							//hasQueuedPredecessors()在父类AbstractQueuedSynchronizer定义
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    }
    
	...

}

可以明显看出公平锁与非公平锁的lock()方法唯一的区别就在于公平锁在获取同步状态时多了一个限制条件：hasQueuedPredecessors()

hasQueuedPredecessors是公平锁加锁时判断等待队列中是否存在有效节点的方法

hasQueuedPredecessors()中判断了是否需要排队，导致公平锁和非公平锁的差异如下：

公平锁：公平锁讲究先来先到，线程在获取锁时，如果这个锁的等待队列中已经有线程在等待，那么当前线程就会进入等待队列中;

非公平锁：不管是否有等待队列，如果可以获取锁，则立刻占有锁对象。也就是说队列的第一个排队线程在unpark()，之后还是需要竞争锁（存在线程竞争的情况下)

接下来讲述非公平锁的lock()

整个ReentrantLock 的加锁过程，可以分为三个阶段：

1. 尝试加锁；
2. 加锁失败，线程入队列；
3. 线程入队列后，进入阻赛状态。

ReentrantLock默认是选用非公平锁。

public class ReentrantLock implements Lock, java.io.Serializable {
    
    ...
        
    * Acquires the lock.
    public void lock() {
        sync.lock();//<------------------------注意，我们从这里入手,线程A
    }
    
    abstract static class Sync extends AbstractQueuedSynchronizer {
        
        ...

        //被NonfairSync的tryAcquire()调用
        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
        ...

    }
    
    
	//非公平锁
	static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {//<----线程A的lock.lock()调用该方法
            if (compareAndSetState(0, 1))//AbstractQueuedSynchronizer的方法,刚开始这方法返回true
                setExclusiveOwnerThread(Thread.currentThread());//设置独占的所有者线程，显然一开始是线程A
            else
                acquire(1);//稍后紧接着的线程B将会调用该方法。
        }

        //acquire()将会间接调用该方法
        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);//调用父类Sync的nonfairTryAcquire()
        }
        

        
    }
    
    ...
}

public class ReentrantLock implements Lock, java.io.Serializable {
    
    ...
        
    * Acquires the lock.
    public void lock() {
        sync.lock();//<------------------------注意，我们从这里入手,线程B
    }
    
	//非公平锁
	static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {//<-------------------------线程B的lock.lock()调用该方法
            if (compareAndSetState(0, 1))//这是预定线程A还在工作，这里返回false
                setExclusiveOwnerThread(Thread.currentThread());//
            else
                acquire(1);//线程B将会调用该方法，该方法在AbstractQueuedSynchronizer，
            			   //它会调用本类的tryAcquire()方法
        }

        //acquire()将会间接调用该方法
        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);//调用父类Sync的nonfairTryAcquire()
        }
    }

    //非公平锁与公平锁的公共父类
     * Base of synchronization control for this lock. Subclassed
    abstract static class Sync extends AbstractQueuedSynchronizer {
    
        //acquire()将会间接调用该方法
    	...
        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();//这里是线程B
            int c = getState();//线程A还在工作，c=>1
            if (c == 0) {//false
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {//(线程B == 线程A) => false
                int nextc = c + acquires;//+1
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;//最终返回false
        } 
        ...
    
    }
    
    ...
}

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {

	...

     * Acquires in exclusive mode, ignoring interrupts.  Implemented
    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&//线程B调用非公平锁的tryAcquire(), 最终返回false，加上!,也就是true,也就是还要执行下面两行语句
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))//下一节论述
            selfInterrupt();
    }
    
    ...
}

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {

	...

     * Acquires in exclusive mode, ignoring interrupts.  Implemented
    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&//线程B调用非公平锁的tryAcquire(), 最终返回false，加上!,也就是true,也就是还要执行下面两行语句
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))//线程B加入等待队列
            selfInterrupt();//下一节论述
    }
    
    private Node addWaiter(Node mode) {
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail;
        if (pred != null) {//根据上面一句注释，本语句块的意义是将新节点快速添加至队尾
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);//快速添加至队尾失败，则用这方法调用（可能链表为空，才调用该方法）
        return node;
    }
    
    //Inserts node into queue, initializing if necessary.
    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))//插入一个哨兵节点（或称傀儡节点）
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {//真正插入我们需要的节点，也就是包含线程B引用的节点
                    t.next = node;
                    return t;
                }
            }
        }
    }
    
    //CAS head field. Used only by enq.
    private final boolean compareAndSetHead(Node update) {
        return unsafe.compareAndSwapObject(this, headOffset, null, update);
    }

    //CAS tail field. Used only by enq.
    private final boolean compareAndSetTail(Node expect, Node update) {
        return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
    }

    
    ...
}

另外

假设线程B，C还没启动，正在工作线程A重新尝试获得锁，也就是调用lock.lock()多一次

    //非公平锁与公平锁的公共父类fa
     * Base of synchronization control for this lock. Subclassed
    abstract static class Sync extends AbstractQueuedSynchronizer {
    
    	...
        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();//这里是线程A
            int c = getState();//线程A还在工作，c=>1；如果线程A恰好运行到在这工作完了，c=>0，这时它又要申请锁的话
            if (c == 0) {//线程A正在工作为false;如果线程A恰好工作完，c=>0，这时它又要申请锁的话,则为true
                if (compareAndSetState(0, acquires)) {//线程A重新获得锁
                    setExclusiveOwnerThread(current);//这里相当于NonfairSync.lock()另一重设置吧！
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {//(线程A == 线程A) => true
                int nextc = c + acquires;//1+1=>nextc=2
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);//state=2,说明要unlock多两次吧（现在盲猜）
                return true;//返回true
            }
            return false;
        } 
        ...
    
    }

双向链表中，第一个节点为虚节点(也叫哨兵节点)，其实并不存储任何信息，只是占位。真正的第一个有数据的节点，是从第二个节点开始的。

接下来讨论ReentrantLock.unLock()方法。假设线程A工作结束，调用unLock()，释放锁占用。

public class ReentrantLock implements Lock, java.io.Serializable {
    
    private final Sync sync;

    abstract static class Sync extends AbstractQueuedSynchronizer {
        
        ...
        // unlock()间接调用本方法，releases传入1
        protected final boolean tryRelease(int releases) {

            int c = getState() - releases;//c为0

            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {//c为0，条件为ture，执行if语句块
                free = true;
                // 设置当前占用锁线程为null
                setExclusiveOwnerThread(null);
            }

            setState(c);
            return free;//最后返回true
        }
    	...
    
    }
    
    static final class NonfairSync extends Sync {...}
    
    public ReentrantLock() {
        sync = new NonfairSync();//我们使用的非公平锁
    }
    					//注意！注意！注意！
    public void unlock() {//<----------从这开始，假设线程A工作结束，调用unLock()，释放锁占用

        sync.release(1);//在AbstractQueuedSynchronizer类定义
    }
    
    ...
 
}

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {

    ...
    //1.
    public final boolean release(int arg) {
        //2.
        if (tryRelease(arg)) {//该方法看子类NonfairSync实现，最后返回true
            Node h = head;//返回傀儡节点
            if (h != null && h.waitStatus != 0)//傀儡节点非空，且状态为-1，条件为true，执行if语句
                //7.
                unparkSuccessor(h);
            return true;
        }
        return false;//返回true,false都无所谓了，unlock方法只是简单调用release方法，对返回结果没要求
    }
    
    /**
     * The synchronization state.
     */
    private volatile int state;

    //3.
    protected final int getState() {
        return state;
    }

    //6.
    protected final void setState(int newState) {
        state = newState;
    }
    
    //7. Wakes up node's successor, if one exists.
    //传入傀儡节点
    private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        int ws = node.waitStatus;//傀儡节点waitStatus为-1
        if (ws < 0)//ws为-1，条件成立，执行if语块
            compareAndSetWaitStatus(node, ws, 0);//8.将傀儡节点waitStatus由-1变为0

        /*
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        Node s = node.next;//傀儡节点的下一节点,也就是带有线程B的节点
        if (s == null || s.waitStatus > 0) {//s非空，s.waitStatus非0，条件为false，不执行if语块
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)//s非空，条件为true，不执行if语块
            LockSupport.unpark(s.thread);//唤醒线程B。运行到这里，线程A的工作基本告一段落了。
    }
    
    //8.
    private static final boolean compareAndSetWaitStatus(Node node,
                                                         int expect,
                                                         int update) {
        return unsafe.compareAndSwapInt(node, waitStatusOffset,
                                        expect, update);
    }
    
    
}

线程A结束工作，调用unlock()的tryRelease()后的状态，state由1变为0，exclusiveOwnerThread由线程A变为null。

线程B被唤醒，即从原先park()的方法继续运行

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {

     private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);//线程B从阻塞到非阻塞，继续执行
        return Thread.interrupted();//线程B没有被中断，返回false
    }
    
	...
 
    //Acquires in exclusive uninterruptible mode for thread already inqueue. 
    //Used by condition wait methods as well as acquire.
    //
    //return true if interrupted while waiting
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();//线程B所在的节点的前一节点是傀儡节点
                //傀儡节点是头节点，tryAcquire()的说明请移步至#21_AQS源码深度解读03
                //tryAcquire()返回true,线程B成功上位
                if (p == head && tryAcquire(arg)) {
                    setHead(node);//1.将附带线程B的节点的变成新的傀儡节点
                    p.next = null; // help GC//置空原傀儡指针与新的傀儡节点之间的前后驱指针，方便GC回收
                    failed = false;
                    return interrupted;//返回false，跳到2.acquire()
                }
               
                if (shouldParkAfterFailedAcquire(p, node) && 
                    //唤醒线程B继续工作，parkAndCheckInterrupt()返回false
                    //if语块不执行，跳到下一循环
                    parkAndCheckInterrupt())//<---------------------------------唤醒线程在这里继续运行
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
    
    //1. 
    private void setHead(Node node) {
        head = node;
        node.thread = null;
        node.prev = null;
    }
    
    //2.
    * Acquires in exclusive mode, ignoring interrupts.  Implemented
    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            //acquireQueued()返回fasle,条件为false，if语块不执行，acquire()返回
            //也就是说，线程B成功获得锁，可以展开线程B自己的工作了。
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();//
    }
    
}

最后，线程B上位成功。

3、spring AOP

springboot1和springboot分别使用了spring4、spring5，对比不同版本aop执行的顺序

3.1、spring4下的aop测试案例

pom文件

    4.0.0
    
        org.springframework.boot
        spring-boot-starter-parent
        1.5.9.RELEASE
         
    
    com.example
    aoptest
    0.0.1-SNAPSHOT
    aoptest
    Demo project for Spring Boot
    
        8
    
    
        
            org.springframework.boot
            spring-boot-starter
        

        
            org.springframework.boot
            spring-boot-starter-test
            test
        
        
            org.aspectj
            aspectjrt
        
        
            org.aspectj
            aspectjweaver
        
    

    
        
            
                org.springframework.boot
                spring-boot-maven-plugin
            
        
    

切面类

package com.example.aoptest.aspect;

import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.*;
import org.springframework.stereotype.Component;

@Aspect
@Component
public class MyAspect {
    @Before("execution(* com.example.aoptest..*ServiceImpl.*(..))")
    public void beforeNotify() {
        System.out.println("********@Before我是前置通知");
    }

    @After("execution(* com.example.aoptest..*ServiceImpl.*(..))")
    public void afterNotify() {
        System.out.println("********@After我是后置通知");
    }

    @AfterReturning("execution(* com.example.aoptest..*ServiceImpl.*(..))")
    public void afterReturningNotify() {
        System.out.println("********@AfterReturning我是返回后通知");
    }

    @AfterThrowing(" execution(* com.example.aoptest..*ServiceImpl.*(..))")
    public void afterThrowingNotify() {
        System.out.println("********@AfterThrowing我是异常通知");
    }

    @Around(" execution(* com.example.aoptest..*ServiceImpl.*(..))")
    public Object around(ProceedingJoinPoint proceedingJoinPoint) throws Throwable {
        Object retvalue = null;
        System.out.println("我是环绕通知之前AAA");
        retvalue = proceedingJoinPoint.proceed();
        System.out.println("我是环绕通知之后BBB");
        return retvalue ;
    }
}

业务service

package com.example.aoptest.service.impl;

import com.example.aoptest.service.HelloService;
import org.springframework.stereotype.Component;

@Component
public class HelloServiceImpl implements HelloService {
    @Override
    public void helloWorld() {
        System.out.println("=========hello world");
    }
}

测试类

package com.example.aoptest;

import com.example.aoptest.service.HelloService;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

import javax.annotation.Resource;

@SpringBootTest
@RunWith(SpringRunner.class)
public class AopTest {

	@Resource
	private HelloService helloService;
	
	@Test
	public void testAop4() {

		helloService.helloWorld();
	}
}

aop执行顺序

我是环绕通知之前AAA
********@Before我是前置通知
=========hello world
我是环绕通知之后BBB
********@After我是后置通知
********@AfterReturning我是返回后通知

业务service发生异常的情况下aop执行顺序

我是环绕通知之前AAA
********@Before我是前置通知
********@After我是后置通知
********@AfterThrowing我是异常通知

java.lang.ArithmeticException: / by zero

3.2、spring5下的aop测试案例

修改pom文件中springboot版本为2.4.4

修改单元测试类

package com.example.aoptest;

import com.example.aoptest.service.HelloService;
import org.junit.jupiter.api.Test;
import org.springframework.boot.test.context.SpringBootTest;

import javax.annotation.Resource;

@SpringBootTest
public class AopTest {

	@Resource
	private HelloService helloService;
	
	@Test
	public void testAop4() {
		helloService.helloWorld();
	}
}

aop执行顺序

我是环绕通知之前AAA
********@Before我是前置通知
=========hello world
********@AfterReturning我是返回后通知
********@After我是后置通知
我是环绕通知之后BBB

业务service发生异常的情况下aop执行顺序

我是环绕通知之前AAA
********@Before我是前置通知
********@AfterThrowing我是异常通知
********@After我是后置通知

java.lang.ArithmeticException: / by zero

4、spring循环依赖

spring默认的单例(Singleton)的场景是支持循环依赖

spring内部通过3级缓存来解决循环依赖 - DefaultSingletonBeanRegistry

只有单例的bean会通过三级缓存提前暴露来解决循环依赖的问题，而非单例的bean，每次从容器中获取都是一个新的对象，都会重新创建，所以非单例的bean是没有缓存的，不会将其放到三级缓存中。

第一级缓存（也叫单例池）singletonObjects：存放已经经历了完整生命周期的Bean对象。

第二级缓存：earlySingletonObjects，存放早期暴露出来的Bean对象，Bean的生命周期未结束（属性还未填充完。

第三级缓存：Map> singletonFactories，存放可以生成Bean的工厂。

4.1、哪些循环依赖Spring无法解决

• prototype 类型的循环依赖
• constructor 注入的循环依赖
• @Async 标记的类产生的代理 bean 发生循环依赖（@Async 标记产生的循环依赖本地可能正常，打包启动报错）

4.2、spring循环依赖debug前置知识

实例化 - 内存中申请一块内存空间，如同租赁好房子，自己的家当还未搬来。

初始化属性填充 - 完成属性的各种赋值，如同装修，家具，家电进场。

3个Map和四大方法，总体相关对象

package org.springframework.beans.factory.support;

...

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {

	...

	/** 
	单例对象的缓存:bean名称—bean实例，即:所谓的单例池。
	表示已经经历了完整生命周期的Bean对象
	第一级缓存
	*/
	private final Map singletonObjects = new ConcurrentHashMap<>(256);

	/**
	早期的单例对象的高速缓存: bean名称—bean实例。
	表示 Bean的生命周期还没走完（Bean的属性还未填充）就把这个 Bean存入该缓存中也就是实例化但未初始化的 bean放入该缓存里
	第二级缓存
	*/
	private final Map earlySingletonObjects = new HashMap<>(16);

	/**
	单例工厂的高速缓存:bean名称—ObjectFactory
	表示存放生成 bean的工厂
	第三级缓存
	*/
	private final Map> singletonFactories = new HashMap<>(16);
 
    ...
    
}

A / B两对象在三级缓存中的迁移说明

• A创建过程中需要B，于是A将自己放到三级缓里面，去实例化B。
• B实例化的时候发现需要A，于是B先查一级缓存，没有，再查二级缓存，还是没有，再查三级缓存，找到了A然后把三级缓存里面的这个A放到二级缓存里面，并删除三级缓存里面的A。
• B顺利初始化完毕，将自己放到一级缓存里面（此时B里面的A依然是创建中状态)，然后回来接着创建A，此时B已经创建结束，直接从一级缓存里面拿到B，然后完成创建，并将A自己放到一级缓存里面。

Spring为了解决单例的循坏依赖问题，使用了三级缓存：

其中一级缓存为单例池(singletonObjects)。

二级缓存为提前曝光对象(earlySingletonObjects)。

三级级存为提前曝光对象工厂(singletonFactories) 。

假设A、B循环引用，实例化A的时候就将其放入三级缓存中，接着填充属性的时候，发现依赖了B，同样的流程也是实例化后放入三级缓存，接着去填充属性时又发现自己依赖A，这时候从缓存中查找到早期暴露的A，没有AOP代理的话，直接将A的原始对象注入B，完成B的初始化后，进行属性填充和初始化，这时候B完成后，就去完成剩下的A的步骤，如果有AOP代理，就进行AOP处理获取代理后的对象A，注入B，走剩下的流程。

4.3 debug循环依赖源码(注解)

4.3.1 源码跟踪

进入SpringApplication.run，源码一

	/**
	 * Run the Spring application, creating and refreshing a new
	 * {@link ApplicationContext}.
	 * @param args the application arguments (usually passed from a Java main method)
	 * @return a running {@link ApplicationContext}
	 */
	public ConfigurableApplicationContext run(String... args) {
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
		DefaultBootstrapContext bootstrapContext = createBootstrapContext();
		ConfigurableApplicationContext context = null;
		configureHeadlessProperty();
		SpringApplicationRunListeners listeners = getRunListeners(args);
		listeners.starting(bootstrapContext, this.mainApplicationClass);
		try {
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
			ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);
			configureIgnoreBeanInfo(environment);
			Banner printedBanner = printBanner(environment);
			context = createApplicationContext();
			context.setApplicationStartup(this.applicationStartup);
			prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
			refreshContext(context); // 这一步创建了循环依赖对象
			afterRefresh(context, applicationArguments);
			stopWatch.stop();
			if (this.logStartupInfo) {
				new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
			}
			listeners.started(context);
			callRunners(context, applicationArguments);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, listeners);
			throw new IllegalStateException(ex);
		}

		try {
			listeners.running(context);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, null);
			throw new IllegalStateException(ex);
		}
		return context;
	}

进入refreshContext(context)，经过几次断点，最终进入refresh()方法，源码二

@Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");

			// Prepare this context for refreshing.
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);
				beanPostProcess.end();

				// Initialize message source for this context.
				initMessageSource();

				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				onRefresh();

				// Check for listener beans and register them.
				registerListeners();

				// 重点关注点是这里
				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory); // 这一步创建了循环依赖对象

				// Last step: publish corresponding event.
				finishRefresh();
			}

			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}

				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exception to caller.
				throw ex;
			}

			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
				contextRefresh.end();
			}
		}
	}

finishBeanFactoryInitialization(beanFactory)，源码三

/**
	 * Finish the initialization of this context's bean factory,
	 * initializing all remaining singleton beans.
	 */
	protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
		// Initialize conversion service for this context.
		if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
				beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
			beanFactory.setConversionService(
					beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
		}

		// Register a default embedded value resolver if no BeanFactoryPostProcessor
		// (such as a PropertySourcesPlaceholderConfigurer bean) registered any before:
		// at this point, primarily for resolution in annotation attribute values.
		if (!beanFactory.hasEmbeddedValueResolver()) {
			beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
		}

		// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
		String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
		for (String weaverAwareName : weaverAwareNames) {
			getBean(weaverAwareName);
		}

		// Stop using the temporary ClassLoader for type matching.
		beanFactory.setTempClassLoader(null);

		// Allow for caching all bean definition metadata, not expecting further changes.
		beanFactory.freezeConfiguration();

		// Instantiate all remaining (non-lazy-init) singletons.
		beanFactory.preInstantiateSingletons(); // 这一步创建了循环依赖对象
	}

进入beanFactory.preInstantiateSingletons()，源码四

@Override
	public void preInstantiateSingletons() throws BeansException {
		if (logger.isTraceEnabled()) {
			logger.trace("Pre-instantiating singletons in " + this);
		}

		// Iterate over a copy to allow for init methods which in turn register new bean definitions.
		// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
		List beanNames = new ArrayList<>(this.beanDefinitionNames);

		// 这里要注意beanNames有很多，找到创建a、b实例的循环，再继续向下debug
		// Trigger initialization of all non-lazy singleton beans...
		for (String beanName : beanNames) {
			RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
			if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
				// 对象类型是不是FactoryBean
				if (isFactoryBean(beanName)) {
					Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
					if (bean instanceof FactoryBean) {
						FactoryBean factory = (FactoryBean) bean;
						boolean isEagerInit;
						if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
							isEagerInit = AccessController.doPrivileged(
									(PrivilegedAction) ((SmartFactoryBean) factory)::isEagerInit,
									getAccessControlContext());
						}
						else {
							isEagerInit = (factory instanceof SmartFactoryBean &&
									((SmartFactoryBean) factory).isEagerInit());
						}
						if (isEagerInit) {
							getBean(beanName);
						}
					}
				}
				else {
					getBean(beanName);
				}
			}
		}

		// Trigger post-initialization callback for all applicable beans...
		for (String beanName : beanNames) {
			Object singletonInstance = getSingleton(beanName);
			if (singletonInstance instanceof SmartInitializingSingleton) {
				StartupStep smartInitialize = this.getApplicationStartup().start("spring.beans.smart-initialize")
						.tag("beanName", beanName);
				SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
				if (System.getSecurityManager() != null) {
					AccessController.doPrivileged((PrivilegedAction