public ConnectionImpl(String hostToConnectTo, int portToConnectTo, Properties info, String databaseToConnectTo, String url) throws SQLException {
    ...
    NonRegisteringDriver.trackConnection(this);
  ...
}

public class NonRegisteringDriver implements Driver {
  ...
 ?protected static final ConcurrentHashMap<ConnectionPhantomReference, ConnectionPhantomReference> connectionPhantomRefs = new ConcurrentHashMap();
  protected static void trackConnection(com.mysql.jdbc.Connection newConn) {
 ? ? ? ?ConnectionPhantomReference phantomRef = new ConnectionPhantomReference((ConnectionImpl)newConn, refQueue);
 ? ? ? ?connectionPhantomRefs.put(phantomRef, phantomRef);
 ?  }
  ...
}

// 每兩小時(shí)清理 connectionPhantomRefs，減少對(duì) mixed GC 的影響
SCHEDULED_EXECUTOR.scheduleAtFixedRate(() -> {
  try {
    Field connectionPhantomRefs = NonRegisteringDriver.class.getDeclaredField("connectionPhantomRefs");
    connectionPhantomRefs.setAccessible(true);
    Map map = (Map) connectionPhantomRefs.get(NonRegisteringDriver.class);
    if (map.size() > 50) {
      map.clear();
    }
  } catch (Exception e) {
    log.error("connectionPhantomRefs clear error!", e);
  }
}, 2, 2, TimeUnit.HOURS);

//靜態(tài)變量通過 System.getProperty 獲取配置
private static boolean abandonedConnectionCleanupDisabled = Boolean.getBoolean("com.mysql.cj.disableAbandonedConnectionCleanup");
public static boolean getBoolean(String name) {
      return parseBoolean(System.getProperty(name));
}
protected static void trackConnection(MysqlConnection conn, NetworkResources io) {
          //判斷配置的屬性值來決定是否需要生成虛引用
      if (!abandonedConnectionCleanupDisabled) {
         ···
          ConnectionFinalizerPhantomReference reference = new ConnectionFinalizerPhantomReference(conn, io, referenceQueue);
          connectionFinalizerPhantomRefs.add(reference);
         ··· 
      }
  }

java -jar app.jar -Dcom.mysql.cj.disableAbandonedConnectionCleanup=true

System.setProperty(PropertyDefinitions.SYSP_disableAbandonedConnectionCleanup,"true");

//虛引用隊(duì)列
ReferenceQueue<Object> queue = new ReferenceQueue<>();
//關(guān)聯(lián)對(duì)象
Object o = new Object();
//調(diào)用構(gòu)造方法生成一個(gè)虛引用 第一個(gè)參數(shù)就是關(guān)聯(lián)對(duì)象 第二個(gè)參數(shù)是關(guān)聯(lián)隊(duì)列
PhantomReference<Object> phantomReference = new PhantomReference<>(o, queue);
//執(zhí)行垃圾回收
System.gc();
//延時(shí)確?；厥胀戤?
Thread.sleep(300L);
//當(dāng) Object o 被回收時(shí)可以從虛引用隊(duì)列里獲取到與之關(guān)聯(lián)的虛引用 這里就是 phantomReference 這個(gè)對(duì)象
Reference<?> poll = queue.poll();

static ReferenceQueue<Object> queue = new ReferenceQueue<>();
public static void main(String[] args) throws InterruptedException {
    PhantomReference<Object> phantomReference = buildReference();
    System.gc();Thread.sleep(100);
    System.out.println(queue.poll());
}
public static PhantomReference<Object> buildReference() {
    Object o = new Object();
    return new PhantomReference<>(o, queue);
}

public static void main(String[] args) {
      ReferenceQueue<byte[]> queue = new ReferenceQueue<>();
      PhantomReference<byte[]> phantomReference = new PhantomReference<>(
              new byte[1024 * 1024 * 2], queue);
      System.gc();Thread.sleep(100L);
      System.out.println(queue.poll());
      byte[] bytes = new byte[1024 * 1024 * 4];
  }

public static void main(String[] args) {
      ReferenceQueue<byte[]> queue = new ReferenceQueue<>();
      PhantomReference<byte[]> phantomReference = new PhantomReference<>(
              new byte[1024 * 1024 * 2], queue);
      System.gc();Thread.sleep(100L);
      System.out.println(queue.poll());
          //虛引用直接置為 null
          phantomReference = null;
      byte[] bytes = new byte[1024 * 1024 * 4];
  }

private static class ConnectionFinalizerPhantomReference extends PhantomReference<MysqlConnection> {
      //放置需要GC后后置處理的網(wǎng)絡(luò)資源
      private NetworkResources networkResources;
      ConnectionFinalizerPhantomReference(MysqlConnection conn, NetworkResources networkResources, ReferenceQueue<? super MysqlConnection> refQueue) {
          super(conn, refQueue);
          this.networkResources = networkResources;
      }
      void finalizeResources() {
          if (this.networkResources != null) {
              try {
                  this.networkResources.forceClose();
              } finally {
                  this.networkResources = null;
              }
          }
      }
  }

public void run() {
    while(true) {
        try {
              ...
            Reference<? extends MysqlConnection> reference = referenceQueue.remove(5000L);
            if (reference != null) {
                  //強(qiáng)轉(zhuǎn)為 ConnectionFinalizerPhantomReference
                finalizeResource((ConnectionFinalizerPhantomReference)reference);
            }
              ...
        }
    }
}
private static void finalizeResource(ConnectionFinalizerPhantomReference reference) {
    try {
          //兜底處理網(wǎng)絡(luò)資源
        reference.finalizeResources();
        reference.clear();
    } finally {
          //移除虛引用 避免可能造成的內(nèi)存溢出
        connectionFinalizerPhantomRefs.remove(reference);
    }
}