Map 四种同步方式的性能比较
如果需要使 Map 线程安全，大致有这么四种方法：
1、使用 synchronized 关键字，这也是最原始的方法。代码如下
Java代码
synchronized(anObject)
{
value = map.get(key);
}
Java代码
synchronized(anObject)
{
value = map.get(key);
}
synchronized(anObject)
{
value = map.get(key);
}
JDK1.2 提供了 Collections.synchronizedMap(originMap) 方法，同步方式其实和上面这段代码相同。
2、使用 JDK1.5 提供的锁（java.util.concurrent.locks.Lock）。代码如下
Java代码
lock.lock();
value = map.get(key);
lock.unlock();
Java代码
lock.lock();
value = map.get(key);
lock.unlock();
lock.lock();
value = map.get(key);
lock.unlock();
3、实际应用中，可能多数操作都是读操作，写操作较少。针对这种情况，可以使用 JDK1.5 提供的读写锁（java.util.concurrent.locks.ReadWriteLock）。代码如下
Java代码
rwlock.readLock().lock();
value = map.get(key);
rwlock.readLock().unlock();
Java代码
rwlock.readLock().lock();
value = map.get(key);
rwlock.readLock().unlock();
rwlock.readLock().lock();
value = map.get(key);
rwlock.readLock().unlock();
这样两个读操作可以同时进行，理论上效率会比方法 2 高。
4、使用 JDK1.5 提供的 java.util.concurrent.ConcurrentHashMap 类。该类将 Map 的存储空间分为若干块，每块拥有自己的锁，大大减少了多个线程争夺同一个锁的情况。代码如下
Java代码
value = map.get(key); //同步机制内置在 get 方法中
Java代码
value = map.get(key); //同步机制内置在 get 方法中
value = map.get(key); //同步机制内置在 get 方法中
写了段测试代码，针对这四种方式进行测试，结果见附图。测试内容为 1 秒钟所有 get 方法调用次数的总和。为了比较，增加了未使用任何同步机制的情况（非安全！）。理论上，不同步应该最快。
我的 CPU 是双核的（Core 2 Duo E6300），因此太多线程也没啥意义，所以只列出了单线程、两个线程和五个线程的情况。更多线程时，CPU 利用率提高，但增加了线程调度的开销，测试结果与五个线程差不多。
从附图可以看出：
1、不同步确实最快，与预期一致。
2、四种同步方式中，ConcurrentHashMap 是最快的，接近不同步的情况。
3、synchronized 关键字非常慢，比使用锁慢了两个数量级。真是大跌眼镜，我很迷惑为什会 synchronized 慢到这个程度。
4、使用读写锁的读锁，比普通所稍慢。这个比较意外，可能硬件或测试代码没有发挥出读锁的全部功效。
结论：
1、如果 ConcurrentHashMap 够用，则使用 ConcurrentHashMap。
2、如果需自己实现同步，则使用 JDK1.5 提供的锁机制，避免使用 synchronized 关键字。
描述: 多个线程每秒 Map.get 的执行次数之和
大小: 28.8 KB
测试代码。只图方便了，不大 OO，见笑。
Java代码
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.locks.*;
public class MapTest
{
public static final int THREAD_COUNT = 1;
public static final int MAP_SIZE = 1000;
public static final int EXECUTION_MILLES = 1000;
public static final int[] KEYS = new int[100];
public static void main(String[] args) throws Exception
{
//初始化
Random rand = new Random();
for (int i = 0; i < KEYS.length; ++i)
{
KEYS[i] = rand.nextInt();
}
//创建线程
long start = System.currentTimeMillis();
Thread[] threads = new Thread[THREAD_COUNT];
for (int i = 0; i < THREAD_COUNT; ++i)
{
//threads[i] = new UnsafeThread();
//threads[i] = new SynchronizedThread();
//threads[i] = new LockThread();
//threads[i] = new ReadLockThread();
threads[i] = new ConcurrentThread();
threads[i].start();
}
//等待其它线程执行若干时间
Thread.sleep(EXECUTION_MILLES);
//统计 get 操作的次数
long sum = 0;
for (int i = 0; i < THREAD_COUNT; ++i)
{
sum += threads[i].getClass().getDeclaredField("count").getLong(threads[i]);
}
long millisCost = System.currentTimeMillis() - start;
System.out.println(sum + "(" + (millisCost) + "ms)");
System.exit(0);
}
public static void fillMap(Map<Integer, Integer> map)
{
Random rand = new Random();
for (int i = 0; i < MAP_SIZE; ++i)
{
map.put(rand.nextInt(), rand.nextInt());
}
}
}
class UnsafeThread extends Thread
{
private static Map<Integer, Integer> map = new HashMap<Integer, Integer>();
public long count = 0;
static
{
MapTest.fillMap(map);
}
public void run()
{
for (;;)
{
int index = (int)(count % MapTest.KEYS.length);
map.get(MapTest.KEYS[index]);
++count;
}
}
}
class SynchronizedThread extends Thread
{
private static Map<Integer, Integer> map = new HashMap<Integer, Integer>();
public long count = 0;
static
{
MapTest.fillMap(map);
}
public void run()
{
for (;;)
{
int index = (int)(count % MapTest.KEYS.length);
synchronized(SynchronizedThread.class)
{
map.get(MapTest.KEYS[index]);
}
++count;
}
}
}
class LockThread extends Thread
{
private static Map<Integer, Integer> map = new HashMap<Integer, Integer>();
private static Lock lock = new ReentrantLock();
public long count = 0;
static
{
MapTest.fillMap(map);
}
public void run()
{
for (;;)
{
int index = (int)(count % MapTest.KEYS.length);
lock.lock();
map.get(MapTest.KEYS[index]);
lock.unlock();
++count;
}
}
}
class ReadLockThread extends Thread
{
private static Map<Integer, Integer> map = new HashMap<Integer, Integer>();
private static Lock lock = new ReentrantReadWriteLock().readLock();
public long count = 0;
static
{
MapTest.fillMap(map);
}
public void run()
{
for (;;)
{
int index = (int)(count % MapTest.KEYS.length);
lock.lock();
map.get(MapTest.KEYS[index]);
lock.unlock();
++count;
}
}
}
class ConcurrentThread extends Thread
{
private static Map<Integer, Integer> map = new ConcurrentHashMap<Integer, Integer>();
public long count = 0;
static
{
MapTest.fillMap(map);
}
public void run()
{
for (;;)
{
int index = (int)(count % MapTest.KEYS.length);
map.get(MapTest.KEYS[index]);
++count;
}
}
}
Java代码
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.locks.*;
public class MapTest
{
public static final int THREAD_COUNT = 1;
public static final int MAP_SIZE = 1000;
public static final int EXECUTION_MILLES = 1000;
public static final int[] KEYS = new int[100];
public static void main(String[] args) throws Exception
{
//初始化
Random rand = new Random();
for (int i = 0; i < KEYS.length; ++i)
{
KEYS[i] = rand.nextInt();
}
//创建线程
long start = System.currentTimeMillis();
Thread[] threads = new Thread[THREAD_COUNT];
for (int i = 0; i < THREAD_COUNT; ++i)
{
//threads[i] = new UnsafeThread();
//threads[i] = new SynchronizedThread();
//threads[i] = new LockThread();
//threads[i] = new ReadLockThread();
threads[i] = new ConcurrentThread();
threads[i].start();
}
//等待其它线程执行若干时间
Thread.sleep(EXECUTION_MILLES);
//统计 get 操作的次数
long sum = 0;
for (int i = 0; i < THREAD_COUNT; ++i)
{
sum += threads[i].getClass().getDeclaredField("count").getLong(threads[i]);
}
long millisCost = System.currentTimeMillis() - start;
System.out.println(sum + "(" + (millisCost) + "ms)");
System.exit(0);
}
public static void fillMap(Map<Integer, Integer> map)
{
Random rand = new Random();
for (int i = 0; i < MAP_SIZE; ++i)
{
map.put(rand.nextInt(), rand.nextInt());
}
}
}
class UnsafeThread extends Thread
{
private static Map<Integer, Integer> map = new HashMap<Integer, Integer>();
public long count = 0;
static
{
MapTest.fillMap(map);
}
public void run()
{
for (;;)
{
int index = (int)(count % MapTest.KEYS.length);
map.get(MapTest.KEYS[index]);
++count;
}
}
}
class SynchronizedThread extends Thread
{
private static Map<Integer, Integer> map = new HashMap<Integer, Integer>();
public long count = 0;
static
{
MapTest.fillMap(map);
}
public void run()
{
for (;;)
{
int index = (int)(count % MapTest.KEYS.length);
synchronized(SynchronizedThread.class)
{
map.get(MapTest.KEYS[index]);
}
++count;
}
}
}
class LockThread extends Thread
{
private static Map<Integer, Integer> map = new HashMap<Integer, Integer>();
private static Lock lock = new ReentrantLock();
public long count = 0;
static
{
MapTest.fillMap(map);
}
public void run()
{
for (;;)
{
int index = (int)(count % MapTest.KEYS.length);
lock.lock();
map.get(MapTest.KEYS[index]);
lock.unlock();
++count;
}
}
}
class ReadLockThread extends Thread
{
private static Map<Integer, Integer> map = new HashMap<Integer, Integer>();
private static Lock lock = new ReentrantReadWriteLock().readLock();
public long count = 0;
static
{
MapTest.fillMap(map);
}
public void run()
{
for (;;)
{
int index = (int)(count % MapTest.KEYS.length);
lock.lock();
map.get(MapTest.KEYS[index]);
lock.unlock();
++count;
}
}
}
class ConcurrentThread extends Thread
{
private static Map<Integer, Integer> map = new ConcurrentHashMap<Integer, Integer>();
public long count = 0;
static
{
MapTest.fillMap(map);
}
public void run()
{
for (;;)
{
int index = (int)(count % MapTest.KEYS.length);
map.get(MapTest.KEYS[index]);
++count;
}
}
}
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.locks.*;
public class MapTest
{
public static final int THREAD_COUNT = 1;
public static final int MAP_SIZE = 1000;
public static final int EXECUTION_MILLES = 1000;
public static final int[] KEYS = new int[100];
public static void main(String[] args) throws Exception
{
//初始化
Random rand = new Random();
for (int i = 0; i < KEYS.length; ++i)
{
KEYS[i] = rand.nextInt();
}
//创建线程
long start = System.currentTimeMillis();
Thread[] threads = new Thread[THREAD_COUNT];
for (int i = 0; i < THREAD_COUNT; ++i)
{
//threads[i] = new UnsafeThread();
//threads[i] = new SynchronizedThread();
//threads[i] = new LockThread();
//threads[i] = new ReadLockThread();
threads[i] = new ConcurrentThread();
threads[i].start();
}
//等待其它线程执行若干时间
Thread.sleep(EXECUTION_MILLES);
//统计 get 操作的次数
long sum = 0;
for (int i = 0; i < THREAD_COUNT; ++i)
{
sum += threads[i].getClass().getDeclaredField("count").getLong(threads[i]);
}
long millisCost = System.currentTimeMillis() - start;
System.out.println(sum + "(" + (millisCost) + "ms)");
System.exit(0);
}
public static void fillMap(Map<Integer, Integer> map)
{
Random rand = new Random();
for (int i = 0; i < MAP_SIZE; ++i)
{
map.put(rand.nextInt(), rand.nextInt());
}
}
}
class UnsafeThread extends Thread
{
private static Map<Integer, Integer> map = new HashMap<Integer, Integer>();
public long count = 0;
static
{
MapTest.fillMap(map);
}
public void run()
{
for (;;)
{
int index = (int)(count % MapTest.KEYS.length);
map.get(MapTest.KEYS[index]);
++count;
}
}
}
class SynchronizedThread extends Thread
{
private static Map<Integer, Integer> map = new HashMap<Integer, Integer>();
public long count = 0;
static
{
MapTest.fillMap(map);
}
public void run()
{
for (;;)
{
int index = (int)(count % MapTest.KEYS.length);
synchronized(SynchronizedThread.class)
{
map.get(MapTest.KEYS[index]);
}
++count;
}
}
}
class LockThread extends Thread
{
private static Map<Integer, Integer> map = new HashMap<Integer, Integer>();
private static Lock lock = new ReentrantLock();
public long count = 0;
static
{
MapTest.fillMap(map);
}
public void run()
{
for (;;)
{
int index = (int)(count % MapTest.KEYS.length);
lock.lock();
map.get(MapTest.KEYS[index]);
lock.unlock();
++count;
}
}
}
class ReadLockThread extends Thread
{
private static Map<Integer, Integer> map = new HashMap<Integer, Integer>();
private static Lock lock = new ReentrantReadWriteLock().readLock();
public long count = 0;
static
{
MapTest.fillMap(map);
}
public void run()
{
for (;;)
{
int index = (int)(count % MapTest.KEYS.length);
lock.lock();
map.get(MapTest.KEYS[index]);
lock.unlock();
++count;
}
}
}
class ConcurrentThread extends Thread
{
private static Map<Integer, Integer> map = new ConcurrentHashMap<Integer, Integer>();
public long count = 0;
static
{
MapTest.fillMap(map);
}
public void run()
{
for (;;)
{
int index = (int)(count % MapTest.KEYS.length);
map.get(MapTest.KEYS[index]);
++count;
}
}
}
大小: 28.8 KB
