序

JCTools是一款对jdk并发数据结构进行增强的并发工具，主要提供了map以及queue的增强数据结构。原来netty还是自己写的MpscLinkedQueueNode，后来新版本就换成使用JCTools的并发队列了。

增强map

• ConcurrentAutoTable(后面几个map/set结构的基础)
• NonBlockingHashMap
• NonBlockingHashMapLong
• NonBlockingHashSet
• NonBlockingIdentityHashMap
• NonBlockingSetInt

增强队列

• SPSC - Single Producer Single Consumer (Wait Free, bounded and unbounded)
• MPSC - Multi Producer Single Consumer (Lock less, bounded and unbounded)
• SPMC - Single Producer Multi Consumer (Lock less, bounded)
• MPMC - Multi Producer Multi Consumer (Lock less, bounded)

maven

    <dependency>
            <groupId>org.jctools</groupId>
            <artifactId>jctools-core</artifactId>
            <version>2.1.0</version>
        </dependency>

ConcurrentAutoTable

替代AtomicLong，专门为高性能的counter设计的。只有几个方法

public void add( long x );
public void decrement();
public void increment();
public void set( long x );
public long get();
public int  intValue();
public long longValue();
public long estimate_get();

对比AtomicLong主要是操作之后没有立即返回

public final long incrementAndGet();
public final long decrementAndGet()

NonBlockingHashMap

• NonBlockingHashMap是对ConcurrentHashMap的增强，对多CPU的支持以及高并发更新提供更好的性能。
• NonBlockingHashMapLong是key为Long型的NonBlockingHashMap。
• NonBlockingHashSet是对NonBlockingHashMap的简单包装以支持set的接口。
• NonBlockingIdentityHashMap是从NonBlockingHashMap改造来的，使用System.identityHashCode()来计算哈希
• NonBlockingSetInt是一个使用CAS的简单的bit-vector

原来是

// --- hash ----------------------------------------------------------------
  // Helper function to spread lousy hashCodes.  Throws NPE for null Key, on
  // purpose - as the first place to conveniently toss the required NPE for a
  // null Key.
  private static final int hash(final Object key) {
    int h = key.hashCode();     // The real hashCode call
    h ^= (h>>>20) ^ (h>>>12);
    h ^= (h>>> 7) ^ (h>>> 4);
    h += h<<7; // smear low bits up high, for hashcodes that only differ by 1
    return h;
  }

改为

// --- hash ----------------------------------------------------------------
  // Helper function to spread lousy hashCodes
  private static final int hash(final Object key) {
    int h = System.identityHashCode(key); // The real hashCode call
    // I assume that System.identityHashCode is well implemented with a good
    // spreader, and a second bit-spreader is redundant.
    //h ^= (h>>>20) ^ (h>>>12);
    //h ^= (h>>> 7) ^ (h>>> 4);
    return h;
  }

doc

• JCTools
• Java Concurrent Counters By Numbers
• Lock Free Queues Index
• 135 Million messages a second between processes in pure Java