怎么才能安全地共享对象？
上章介绍对象的发布，这章将介绍一下发布的对象如何让其安全地共享。
内容： 不可变对象， 线程封闭， 同步容器
不可变对象
条件
final关键字
修饰变量： 基本类型和引用类型
package com.accat.concurrency.example.immutable;
import com.google.common.collect.Maps;
import com.accat.concurrency.annoations.NotThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.Map;
@Slf4j
@NotThreadSafe
public class ImmutableExample1 {
private final static Integer a = 1;
private final static String b = "2";
private final static Map map = Maps.newHashMap();
static {
map.put(1, 2);
map.put(3, 4);
map.put(5, 6);
}
public static void main(String[] args) {
// a = 2;
// b = "3";
// map = Maps.newHashMap();
map.put(1, 3);
log.info("{}", map.get(1));
}
private void test(final int a) {
// a = 1;
}
}
不可增加删除的数据集（List， Set， Map）
Collections中提供的不可变集合
package com.accat.concurrency.example.immutable;
import com.google.common.collect.Maps;
import com.accat.concurrency.annoations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.Collections;
import java.util.Map;
@Slf4j
@ThreadSafe
public class ImmutableExample2 {
private static Map map = Maps.newHashMap();
static {
map.put(1, 2);
map.put(3, 4);
map.put(5, 6);
map = Collections.unmodifiableMap(map);
}
public static void main(String[] args) {
map.put(1, 3);
log.info("{}", map.get(1));
}
}
package com.accat.concurrency.example.immutable;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableSet;
import com.accat.concurrency.annoations.ThreadSafe;
@ThreadSafe
public class ImmutableExample3 {
private final static ImmutableList list = ImmutableList.of(1, 2, 3);
private final static ImmutableSet set = ImmutableSet.copyOf(list);
private final static ImmutableMap map = ImmutableMap.of(1, 2, 3, 4);
private final static ImmutableMap map2 = ImmutableMap.builder()
.put(1, 2).put(3, 4).put(5, 6).build();
public static void main(String[] args) {
System.out.println(map2.get(3));
}
}
线程封闭
把对象封装到一个线程里，只有这个线程能看到该对象，从而实现线程安全。
重点
使用ThreadLocal封装一个全局的请求参数对象
package com.accat.concurrency.example.threadLocal;
public class RequestHolder {
private final static ThreadLocalpackage com.accat.concurrency;
import com.accat.concurrency.example.threadLocal.RequestHolder;
import lombok.extern.slf4j.Slf4j;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import java.io.IOException;
@Slf4j
public class HttpFilter implements Filter {
@Override
public void init(FilterConfig filterConfig) throws ServletException {
}
@Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest) servletRequest;
// log.info("do filter, {}, {}", Thread.currentThread().getId(), request.getServletPath());
RequestHolder.add(Thread.currentThread().getParameterMap());
filterChain.doFilter(servletRequest, servletResponse);
}
@Override
public void destroy() {
}
}
package com.accat.concurrency;
import com.accat.concurrency.example.threadLocal.RequestHolder;
import lombok.extern.slf4j.Slf4j;
import org.springframework.web.servlet.handler.HandlerInterceptorAdapter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@Slf4j
public class HttpInterceptor extends HandlerInterceptorAdapter {
@Override
public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
// log.info("preHandle");
return true;
}
@Override
public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
RequestHolder.remove();
// log.info("afterCompletion");
return;
}
}
package com.accat.concurrency.example.threadLocal;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;
@Controller
@RequestMapping("/threadLocal")
public class ThreadLocalController {
@RequestMapping("/test")
@ResponseBody
public Object test() {
return RequestHolder.getParameterMap();
}
}
同步容器
当对象即为可变共享对象，又没有使用线程封闭技术，那么就需要将对象转变为线程安全类。如：
StringBuilder->StringBuffer
java.text.SimpleDateFormat->org.joda.time.DateTimeFormatter
ArrayList->Vector
HashMap->HashTable(key, value 不能为null)
Collections.synchronizedXXX(List, Set, Map)
private static List list = Collections.synchronizedList(Lists.newArrayList());
private static Set set = Collections.synchronizedSet(Sets.newHashSet());
private static Map map = Collections.synchronizedMap(new HashMap<>());
并发容器
并发容器
并发容器在JDK提供的一组并发集合包，JUC。
CopyOnWriteArrayList读写分离，最终一致性，另外开辟空间， 读不加锁，写加锁
package com.accat.concurrency.example.concurrent;
import com.accat.concurrency.annoations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.List;
import java.util.concurrent.*;
@Slf4j
@ThreadSafe
public class CopyOnWriteArrayListExample {
// 请求总数
public static int clientTotal = 5000;
// 同时并发执行的线程数
public static int threadTotal = 200;
private static List list = new CopyOnWriteArrayList<>();
public static void main(String[] args) throws Exception {
ExecutorService executorService = Executors.newCachedThreadPool();
final Semaphore semaphore = new Semaphore(threadTotal);
final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
for (int i = 0; i < clientTotal; i++) {
final int count = i;
executorService.execute(() -> {
try {
semaphore.acquire();
update(count);
semaphore.release();
} catch (Exception e) {
log.error("exception", e);
}
countDownLatch.countDown();
});
}
countDownLatch.await();
executorService.shutdown();
log.info("size:{}", list.size());
}
private static void update(int i) {
list.add(i);
}
}
/**
* {@inheritDoc}
*
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
return get(getArray(), index);
}
/**
* Replaces the element at the specified position in this list with the
* specified element.
*
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E set(int index, E element) {
final ReentrantLock lock = this.lock;
lock.lock();
try {
Object[] elements = getArray();
E oldValue = get(elements, index);
if (oldValue != element) {
int len = elements.length;
Object[] newElements = Arrays.copyOf(elements, len);
newElements[index] = element;
setArray(newElements);
} else {
// Not quite a no-op; ensures volatile write semantics
setArray(elements);
}
return oldValue;
} finally {
lock.unlock();
}
}
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return {@code true} (as specified by {@link Collection#add})
*/
public boolean add(E e) {
final ReentrantLock lock = this.lock;
lock.lock();
try {
Object[] elements = getArray();
int len = elements.length;
Object[] newElements = Arrays.copyOf(elements, len + 1);
newElements[len] = e;
setArray(newElements);
return true;
} finally {
lock.unlock();
}
}
CopyOnWriteArrayList
相似的结构有
CopyOnWriteSet
package com.accat.concurrency.example.concurrent;
import com.accat.concurrency.annoations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.Set;
import java.util.concurrent.ConcurrentSkipListSet;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
@ThreadSafe
public class ConcurrentSkipListSetExample {
// 请求总数
public static int clientTotal = 5000;
// 同时并发执行的线程数
public static int threadTotal = 200;
private static Set set = new ConcurrentSkipListSet<>();
public static void main(String[] args) throws Exception {
ExecutorService executorService = Executors.newCachedThreadPool();
final Semaphore semaphore = new Semaphore(threadTotal);
final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
for (int i = 0; i < clientTotal; i++) {
final int count = i;
executorService.execute(() -> {
try {
semaphore.acquire();
update(count);
semaphore.release();
} catch (Exception e) {
log.error("exception", e);
}
countDownLatch.countDown();
});
}
countDownLatch.await();
executorService.shutdown();
log.info("size:{}", set.size());
}
private static void update(int i) {
set.add(i);
}
}
ConcurrentSkipListSet
package com.accat.concurrency.example.concurrent;
import com.accat.concurrency.annoations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
@ThreadSafe
public class ConcurrentHashMapExample {
// 请求总数
public static int clientTotal = 5000;
// 同时并发执行的线程数
public static int threadTotal = 200;
private static Map map = new ConcurrentHashMap<>();
public static void main(String[] args) throws Exception {
ExecutorService executorService = Executors.newCachedThreadPool();
final Semaphore semaphore = new Semaphore(threadTotal);
final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
for (int i = 0; i < clientTotal; i++) {
final int count = i;
executorService.execute(() -> {
try {
semaphore.acquire();
update(count);
semaphore.release();
} catch (Exception e) {
log.error("exception", e);
}
countDownLatch.countDown();
});
}
countDownLatch.await();
executorService.shutdown();
log.info("size:{}", map.size());
}
private static void update(int i) {
map.put(i, i);
}
}
package com.mmall.concurrency.example.concurrent;
import com.mmall.concurrency.annoations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.Map;
import java.util.concurrent.*;
@Slf4j
@ThreadSafe
public class ConcurrentSkipListMapExample {
// 请求总数
public static int clientTotal = 5000;
// 同时并发执行的线程数
public static int threadTotal = 200;
private static Map map = new ConcurrentSkipListMap<>();
public static void main(String[] args) throws Exception {
ExecutorService executorService = Executors.newCachedThreadPool();
final Semaphore semaphore = new Semaphore(threadTotal);
final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
for (int i = 0; i < clientTotal; i++) {
final int count = i;
executorService.execute(() -> {
try {
semaphore.acquire();
update(count);
semaphore.release();
} catch (Exception e) {
log.error("exception", e);
}
countDownLatch.countDown();
});
}
countDownLatch.await();
executorService.shutdown();
log.info("size:{}", map.size());
}
private static void update(int i) {
map.put(i, i);
}
}
ConcurrentHashMap和ConcurrentSkipListMap的区别在于
前者性能更好，4个线程，1.6万数据量下，前者的性能是后者的4倍。
但是后者是有序的，前者做不到有序。
总结1
总结2