关于common pool 对象池的使用

1 继承工厂类 BasePooledObjectFactory

public class ChannelPoolFactory extends BasePooledObjectFactory<ManagedChannel> {

    private String host;

    private int port;

    private String domain;

    private File cert;

    public ChannelPoolFactory(String host, int port, String domain, String cert) {
        this.host = host;
        this.port = port;
        this.domain = domain;
        URL url = ChannelPoolFactory.class.getClassLoader().getResource(cert);
        this.cert = new File(url.getFile());
    }

    @Override
    public ManagedChannel create() throws SSLException, UnknownHostException {

        SslContext sslContext  = GrpcSslContexts.forClient().trustManager(cert).build();

        InetAddress address = InetAddress.getByAddress(domain, InetAddress.getByName(host).getAddress());

        ManagedChannel channel = NettyChannelBuilder.forAddress(new InetSocketAddress(address, port))
                .negotiationType(NegotiationType.TLS).sslContext(sslContext).build();
        return channel;
    }

2 生成GenericObjectPoolConfig 和 Factory来构造 ObjectPool ；

 public GenericObjectPool(PooledObjectFactory<T> factory,
            GenericObjectPoolConfig config) {

        super(config, ONAME_BASE, config.getJmxNamePrefix());

        if (factory == null) {
            jmxUnregister(); // tidy up
            throw new IllegalArgumentException("factory may not be null");
        }
        this.factory = factory;

        idleObjects = new LinkedBlockingDeque<PooledObject<T>>(config.getFairness());

        setConfig(config);

        startEvictor(getTimeBetweenEvictionRunsMillis());
    }

3 在 GenericObjectPool 有borrowObject 和 returnObject方法，以借还池中对象。

在borrowObject中

 p = idleObjects.pollFirst();
                if (p == null) {
                    p = create();
                    if (p != null) {
                        create = true;
                    }
                }
                if (p == null) {
                    if (borrowMaxWaitMillis < 0) {
                        p = idleObjects.takeFirst();
                    } else {
                        p = idleObjects.pollFirst(borrowMaxWaitMillis,
                                TimeUnit.MILLISECONDS);
                    }
                }
                if (p == null) {
                    throw new NoSuchElementException(
                            "Timeout waiting for idle object");
                }
                if (!p.allocate()) {
                    p = null;
                }

在create()中有

  try {
            p = factory.makeObject();
        } catch (Exception e) {
            createCount.decrementAndGet();
            throw e;
        }

在PooledObjectFactory中 makeObject（）实现为：

  @Override
    public PooledObject<T> makeObject() throws Exception {
        return wrap(create());
    }

由此可知,如果首次borrowObject，将会调用工厂方法创建对象。

创建成功后将会执行

  updateStatsBorrow(p, System.currentTimeMillis() - waitTime);

        return p.getObject();

final void updateStatsBorrow(PooledObject<T> p, long waitTime) {
        borrowedCount.incrementAndGet();
        idleTimes.add(p.getIdleTimeMillis());
        waitTimes.add(waitTime);

        // lock-free optimistic-locking maximum
        long currentMax;
        do {
            currentMax = maxBorrowWaitTimeMillis.get();
            if (currentMax >= waitTime) {
                break;
            }
        } while (!maxBorrowWaitTimeMillis.compareAndSet(currentMax, waitTime));
    }

此操作将会更新参数。

4 returnObject

方法用来归还对象，此处一定要在finally代码中执行。否则每次执行的时候将会创建新对象，到最大对象数目，程序将出现异常。

@Override
    public void returnObject(T obj) {
        PooledObject<T> p = allObjects.get(new IdentityWrapper<T>(obj));
        
 
        synchronized(p) {
            final PooledObjectState state = p.getState();
            if (state != PooledObjectState.ALLOCATED) {
                throw new IllegalStateException(
                        "Object has already been returned to this pool or is invalid");
            } else {
                p.markReturning(); // Keep from being marked abandoned
            }
        }

        long activeTime = p.getActiveTimeMillis();
   
        try {
            factory.passivateObject(p);
        } catch (Exception e1) {
            swallowException(e1);
              destroy(p);
              ensureIdle(1, false);
              updateStatsReturn(activeTime);
            return;
        }
        int maxIdleSave = getMaxIdle();
        if (isClosed() || maxIdleSave > -1 && maxIdleSave <= idleObjects.size()) {
            try {
                destroy(p);
            } catch (Exception e) {
                swallowException(e);
            }
        } else {
            if (getLifo()) {
                idleObjects.addFirst(p);
            } else {
                idleObjects.addLast(p);
            }
            if (isClosed()) {
                // Pool closed while object was being added to idle objects.
                // Make sure the returned object is destroyed rather than left
                // in the idle object pool (which would effectively be a leak)
                clear();
            }
        }
        updateStatsReturn(activeTime);
    }

从中我们可以看到一些destory object策略。