在我们的系统中，我们将客户端会话呈现为会话类。
从历史上看，此类的哈希码是可变的 - 它在创建时为 0，并在某个时间点更改为用户 ID。
系统中存在两个会话管理器。
- Client Sessions Manager - 保持活跃的客户端会话是客户端。这个管理器内部是简单的 ConcurrentHashMap< Long, Session >，其中键是用户 ID。
- 内存会话管理器 - 保存 GC 尚未收集的会话。里面有WeakHashMap<Session,Long>（这里的值是用户id）
当客户端连接并登录时，下一个流程发生：
- 客户端会话已创建。
- 会话放入内存会话管理器（此时哈希码为 0，因此它们被放入单个存储桶中）
- 客户端已登录并使用正确的哈希码将会话放入客户端会话管理器。
- 客户端会话已关闭并从客户端会话管理器中删除。
因此，在收集了 Session 对象上的所有强引用后，它必须从 WeakHashMap 中删除。（WeakHashMap 中的条目将在其键不再正常使用时自动删除。更准确地说，给定键的映射的存在不会阻止该键被垃圾收集器丢弃，也就是说，使其可终结，最终确定，然后回收。）但由于某种原因，它们被留在那里。它的代码如下：
public class MemorySessionManager implements Runnable {
private static final Logger logger = LoggerFactory.getLogger("checker");
private Map<Session, Long> sessions;
public MemorySessionManager() {
this.sessions = new WeakHashMap<>();
}
public synchronized void addSession(Session sess) {
sessions.put(sess, sess.getId());
}
public void run() {
Set<Session> sessionsToCheck = new HashSet<>();
synchronized (this) {
sessionsToCheck.addAll(sessions.keySet());
}
for (Session sess : sessionsToCheck) {
logger.warn("MemorySessionManager: Is still here: " + sess);
}
}
}
简化的程序流程和会话类（没有无用的信息）。
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
public class ClientSessionManager {
private Map<Long, Session> sessions;
public ClientSessionManager() {
this.sessions = new ConcurrentHashMap<>();
}
public void addSession(Session session) {
sessions.put(session.getUserId(), session);
}
public Session removeSession(long code) {
return sessions.remove(code);
}
}
public class Session {
private long userId;
public void setUserId(long userId) {
this.userId = userId;
}
public long getUserId() {
return userId;
}
@Override
public boolean equals(Object o) {
if (this == o)
return true;
if (o == null || getClass() != o.getClass())
return false;
Session session = (Session) o;
return userId == session.userId;
}
@Override
public int hashCode() {
return (int) (userId ^ (userId >>> 32));
}
}
public class Process {
private static final ClientSessionManager CLIENT_SESSION_MANAGER = new ClientSessionManager();
private static final MemorySessionManager MEMORY_SESSION_MANAGER = new MemorySessionManager();
/**
* Login user, create its session and register session into appropriate managers.
*
* @param userId id of user
*/
public void login(long userId) {
Session session = new Session();
MEMORY_SESSION_MANAGER.addSession(session);
session.setUserId(userId);
CLIENT_SESSION_MANAGER.addSession(session);
}
/**
* Close session of user, remove it from session manager
*
* @param userId id of user
*/
public void close(long userId) {
CLIENT_SESSION_MANAGER.removeSession(userId);
}
}
经过大量的 GC 循环后仍保留在内存中。这是 GC 上的日志（G1，混合）
[GC pause (G1 Evacuation Pause) (mixed)
Desired survivor size 6815744 bytes, new threshold 4 (max 15)
- age 1: 236400 bytes, 236400 total
- age 2: 350240 bytes, 586640 total
- age 3: 3329024 bytes, 3915664 total
- age 4: 2926992 bytes, 6842656 total
, 0.0559520 secs]
[Parallel Time: 51.9 ms, GC Workers: 2]
[GC Worker Start (ms): Min: 73278041.7, Avg: 73278041.8, Max: 73278042.0, Diff: 0.2]
[Ext Root Scanning (ms): Min: 3.2, Avg: 3.4, Max: 3.7, Diff: 0.5, Sum: 6.9]
[Update RS (ms): Min: 15.0, Avg: 15.0, Max: 15.0, Diff: 0.0, Sum: 30.1]
[Processed Buffers: Min: 59, Avg: 65.0, Max: 71, Diff: 12, Sum: 130]
[Scan RS (ms): Min: 13.6, Avg: 14.3, Max: 15.1, Diff: 1.5, Sum: 28.7]
[Code Root Scanning (ms): Min: 0.4, Avg: 1.2, Max: 2.0, Diff: 1.5, Sum: 2.4]
[Object Copy (ms): Min: 17.5, Avg: 17.7, Max: 17.8, Diff: 0.3, Sum: 35.4]
[Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
[Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 2]
[GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
[GC Worker Total (ms): Min: 51.6, Avg: 51.7, Max: 51.8, Diff: 0.2, Sum: 103.4]
[GC Worker End (ms): Min: 73278093.6, Avg: 73278093.6, Max: 73278093.6, Diff: 0.0]
[Code Root Fixup: 0.7 ms]
[Code Root Purge: 0.0 ms]
[Clear CT: 0.4 ms]
[Other: 2.9 ms]
[Choose CSet: 0.6 ms]
[Ref Proc: 0.3 ms]
[Ref Enq: 0.0 ms]
[Redirty Cards: 0.3 ms]
[Humongous Register: 0.1 ms]
[Humongous Reclaim: 0.0 ms]
[Free CSet: 0.9 ms]
[Eden: 95.0M(95.0M)->0.0B(1223.0M) Survivors: 7168.0K->5120.0K Heap: 512.0M(2048.0M)->363.5M(2048.0M)]
[Times: user=0.11 sys=0.00, real=0.06 secs]
2016-09-28T10:40:00.815+0000: 73288.384: [GC pause (G1 Evacuation Pause) (young)
Desired survivor size 80740352 bytes, new threshold 15 (max 15)
- age 1: 1096960 bytes, 1096960 total
- age 2: 220208 bytes, 1317168 total
- age 3: 349352 bytes, 1666520 total
- age 4: 3325200 bytes, 4991720 tota
在下图中，您可以看到问题会话的根路径，并且您可以看到只存在弱引用链。
请帮忙。或者至少给出一些建议。