其他的不可变对象的创建
/**
* 初始化的时候将传进来的map赋值给一个final类型的map,然后将所有会修改的方法直接抛出UnsupportedOperationException异常
* Returns an unmodifiable view of the specified map. This method
* allows modules to provide users with "read-only" access to internal
* maps. Query operations on the returned map "read through"
* to the specified map, and attempts to modify the returned
* map, whether direct or via its collection views, result in an
* <tt>UnsupportedOperationException</tt>.<p>
*
* The returned map will be serializable if the specified map
* is serializable.
*
* @param <K> the class of the map keys
* @param <V> the class of the map values
* @param m the map for which an unmodifiable view is to be returned.
* @return an unmodifiable view of the specified map.
*/
public static <K,V> Map<K,V> unmodifiableMap(Map<? extends K, ? extends V> m) {
return new UnmodifiableMap<>(m);
}
/**
* @serial include
*/
private static class UnmodifiableMap<K,V> implements Map<K,V>, Serializable {
private static final long serialVersionUID = -1034234728574286014L;
private final Map<? extends K, ? extends V> m;
UnmodifiableMap(Map<? extends K, ? extends V> m) {
if (m==null)
throw new NullPointerException();
this.m = m;
}
public int size() {return m.size();}
public boolean isEmpty() {return m.isEmpty();}
public boolean containsKey(Object key) {return m.containsKey(key);}
public boolean containsValue(Object val) {return m.containsValue(val);}
public V get(Object key) {return m.get(key);}
public V put(K key, V value) {
throw new UnsupportedOperationException();
}
public V remove(Object key) {
throw new UnsupportedOperationException();
}
测试
@ThreadSafe
public class ImmutableExample1 {
private static Map<Integer,Integer> map = Maps.newHashMap();
static {
map.put(1,2);
map = Collections.unmodifiableMap(map);
}
public static void main(String[] args) {
//Exception in thread "main" java.lang.UnsupportedOperationException
// at java.util.Collections$UnmodifiableMap.put(Collections.java:1457)
// at com.gwf.concurrency.example.immutable.ImmutableExample1.main(ImmutableExample1.java:21)
map.put(1,3);
}
}
// ImmutableList
public static <E> ImmutableList<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10, E e11) {
return construct(e1, e2, e3, e4, e5, e6, e7, e8, e9, e10, e11);
}
// 超过12个元素,则声明为一个数组
public static <E> ImmutableList<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10, E e11, E e12, E... others) {
Object[] array = new Object[12 + others.length];
array[0] = e1;
array[1] = e2;
array[2] = e3;
array[3] = e4;
array[4] = e5;
array[5] = e6;
array[6] = e7;
array[7] = e8;
array[8] = e9;
array[9] = e10;
array[10] = e11;
array[11] = e12;
System.arraycopy(others, 0, array, 12, others.length);
return construct(array);
}
private static <E> ImmutableList<E> construct(Object... elements) {
for(int i = 0; i < elements.length; ++i) {
ObjectArrays.checkElementNotNull(elements[i], i);
}
return new RegularImmutableList(elements);
}
实例
@ThreadSafe
public class ImmutableExample2 {
private final static List<Integer> list = ImmutableList.of(1,2,3);
private final static ImmutableSet set = ImmutableSet.copyOf(list);
// 奇数位参数为key,偶数位参数为value
private final static ImmutableMap map1 = ImmutableMap.of(1,2,3,5);
private final static ImmutableMap<Integer,Integer> map2 = ImmutableMap.<Integer,Integer>builder()
.put(1,2).put(3,4).build();
public static void main(String[] args) {
// 执行都会跑出 UnsupportedOperationException异常
// 但是使用ImmutableXXX声明会直接在编译的时候就告诉你这个方法已经被废弃
list.add(5);
set.add(6);
map1.put(1,2);
map2.put(3,4);
}
}
把对象封装到一个线程里,只有这个线程能看到这个对象
ThreadLocal 实例保存登录用户信息 (具体的业务场景,和拦截器的使用就不赘述了,大家可以购买课程详细学习)
public class RequestHolder {
private final static ThreadLocal<Long> requestHolder = new ThreadLocal<>();
/**
* 添加数据
* 在filter里将登录用户信息存入ThreadLocal
* 如果不使用ThreadLocal,我们会需要将request一直透传
* @param id
*/
public static void add(Long id){
// ThreadLocal 内部维护一个map,key为当前线程名,value为当前set的变量
requestHolder.set(id);
}
/**
* 获取数据
* @return
*/
public static Long getId(){
return requestHolder.get();
}
/**
* 移除变量信息
* 如果不移除,那么变量不会释放掉,会造成内存泄漏
* 在接口处理完以后进行处理(interceptor)
*/
public static void remove(){
requestHolder.remove();
}
}
1.StringBuilder 线程不安全,StringBuffer线程安全 原因:StringBuffer几乎所有的方法都加了synchronized关键字
/**
* 由于StringBuffer 加了 synchronized 所以性能会下降很多
* 所以在堆栈封闭等线程安全的环境下应该首先选用StringBuilder
*/
@Override
public synchronized StringBuffer append(Object obj) {
toStringCache = null;
super.append(String.valueOf(obj));
return this;
}
2.SimpleDateFormat SimpleDateFormat 在多线程共享使用的时候回抛出转换异常,应该才用堆栈封闭在每次调用方法的时候在方法里创建一个SimpleDateFormat 另一种方式是使用joda-time的DateTimeFormatter(推荐使用)
<dependency>
<groupId>joda-time</groupId>
<artifactId>joda-time</artifactId>
<version>2.9</version>
</dependency>
private static DateTimeFormatter dateTimeFormatter = DateTimeFormat.forPattern("yyyyMMdd");
DateTime.parse("20180320",dateTimeFormatter).toDate();
3.ArrayList,HashMap,HashSet等Collections
4.先检查再执行
// 非原子性
if(condition(a)){
handle(a);
}
第一类
vector的所有方法都是有synchronized关键字保护的 stack继承了vector,并且提供了栈操作(先进后出) hashtable也是由synchronized关键字保护
注意:1.同步容器并不一定线程安全
/**
* 并发测试
* 同步容器不一定线程安全
* @author gaowenfeng
*/
@Slf4j
@NotThreadSafe
public class VectorExample2 {
/** 请求总数 */
public static int clientTotal = 5000;
/** 同时并发执行的线程数 */
public static int threadTotal = 50;
public static List<Integer> list = new Vector<>();
public static void main(String[] args) throws InterruptedException {
for (int i = 0; i < 10; i++) {
list.add(i);
}
Thread thread1 = new Thread(() -> {
for (int i = 0; i < 10; i++) {
list.remove(i);
}
});
Thread thread2 = new Thread(() -> {
// thread2想获取i=9的元素的时候,thread1将i=9的元素移除了,导致数组越界
for (int i = 0; i < 10; i++) {
list.get(i);
}
});
thread1.start();
thread2.start();
}
}
注意:2.在foreach或迭代器遍历的过程中不要做删除操作,应该先标记,然后最后再统一删除
public class VectorExample3 {
// java.util.ConcurrentModificationException
// 在遍历的同时进行了删除的操作,导致抛出了并发修改的异常
private static void test1(Vector<Integer> v1) { // foreach
for(Integer i : v1) {
if (i.equals(3)) {
v1.remove(i);
}
}
}
// java.util.ConcurrentModificationException
private static void test2(Vector<Integer> v1) { // iterator
Iterator<Integer> iterator = v1.iterator();
while (iterator.hasNext()) {
Integer i = iterator.next();
if (i.equals(3)) {
v1.remove(i);
}
}
}
// success
private static void test3(Vector<Integer> v1) { // for
for (int i = 0; i < v1.size(); i++) {
if (v1.get(i).equals(3)) {
v1.remove(i);
}
}
}
public static void main(String[] args) {
Vector<Integer> vector = new Vector<>();
vector.add(1);
vector.add(2);
vector.add(3);
test1(vector);
}
}