ConcurrentHashMap、synchronized与线程安全
2017-11-17 22:42
302 查看
最近做的项目中遇到一个问题:明明用了ConcurrentHashMap,可是始终线程不安全
除去项目中的业务逻辑,
4000
简化后的代码如下:
[java] view
plain copy
public class Test40 {
public static void main(String[] args) throws InterruptedException {
for (int i = 0; i < 10; i++) {
System.out.println(test());
}
}
private static int test() throws InterruptedException {
ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<String, Integer>();
ExecutorService pool = Executors.newCachedThreadPool();
for (int i = 0; i < 8; i++) {
pool.execute(new MyTask(map));
}
pool.shutdown();
pool.awaitTermination(1, TimeUnit.DAYS);
return map.get(MyTask.KEY);
}
}
class MyTask implements Runnable {
public static final String KEY = "key";
private ConcurrentHashMap<String, Integer> map;
public MyTask(ConcurrentHashMap<String, Integer> map) {
this.map = map;
}
@Override
public void run() {
for (int i = 0; i < 100; i++) {
this.addup();
}
}
private void addup() {
if (!map.containsKey(KEY)) {
map.put(KEY, 1);
} else {
map.put(KEY, map.get(KEY) + 1);
}
}
}
测试代码跑了10次,每次都不是800。这就很让人疑惑了,难道ConcurrentHashMap的线程安全性失效了?
查了一些资料后发现,原来ConcurrentHashMap的线程安全指的是,它的每个方法单独调用(即原子操作)都是线程安全的,但是代码总体的互斥性并不受控制。以上面的代码为例,最后一行中的:
[java] view
plain copy
map.put(KEY, map.get(KEY) + 1);
实际上并不是原子操作,它包含了三步:
map.get
加1
map.put
其中第1和第3步,单独来说都是线程安全的,由ConcurrentHashMap保证。但是由于在上面的代码中,map本身是一个共享变量。当线程A执行map.get的时候,其它线程可能正在执行map.put,这样一来当线程A执行到map.put的时候,线程A的值就已经是脏数据了,然后脏数据覆盖了真值,导致线程不安全
简单地说,ConcurrentHashMap的get方法获取到的是此时的真值,但它并不保证当你调用put方法的时候,当时获取到的值仍然是真值
为了使上面的代码变得线程安全,我引入了synchronized关键字来修饰目标方法,如下:
[java] view
plain copy
public class Test40 {
public static void main(String[] args) throws InterruptedException {
for (int i = 0; i < 10; i++) {
System.out.println(test());
}
}
private static int test() throws InterruptedException {
ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<String, Integer>();
ExecutorService pool = Executors.newCachedThreadPool();
for (int i = 0; i < 8; i++) {
pool.execute(new MyTask(map));
}
pool.shutdown();
pool.awaitTermination(1, TimeUnit.DAYS);
return map.get(MyTask.KEY);
}
}
class MyTask implements Runnable {
public static final String KEY = "key";
private ConcurrentHashMap<String, Integer> map;
public MyTask(ConcurrentHashMap<String, Integer> map) {
this.map = map;
}
@Override
public void run() {
for (int i = 0; i < 100; i++) {
this.addup();
}
}
private synchronized void addup() { // 用关键字synchronized修饰addup方法
if (!map.containsKey(KEY)) {
map.put(KEY, 1);
} else {
map.put(KEY, map.get(KEY) + 1);
}
}
}
运行之后仍然是线程不安全的,难道synchronized也失效了?
查阅了synchronized的资料后,原来,不管synchronized是用来修饰方法,还是修饰代码块,其本质都是锁定某一个对象。修饰方法时,锁上的是调用这个方法的对象,即this;修饰代码块时,锁上的是括号里的那个对象
在上面的代码中,很明显就是锁定的MyTask对象本身。但是由于在每一个线程中,MyTask对象都是独立的,这就导致实际上每个线程都对自己的MyTask进行锁定,而并不会干涉其它线程的MyTask对象。换言之,上锁压根没有意义
理解到这点之后,对上面的代码又做了一次修改:
[java] view
plain copy
public class Test40 {
public static void main(String[] args) throws InterruptedException {
for (int i = 0; i < 10; i++) {
System.out.println(test());
}
}
private static int test() throws InterruptedException {
ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<String, Integer>();
ExecutorService pool = Executors.newCachedThreadPool();
for (int i = 0; i < 8; i++) {
pool.execute(new MyTask(map));
}
pool.shutdown();
pool.awaitTermination(1, TimeUnit.DAYS);
return map.get(MyTask.KEY);
}
}
class MyTask implements Runnable {
public static final String KEY = "key";
private ConcurrentHashMap<String, Integer> map;
public MyTask(ConcurrentHashMap<String, Integer> map) {
this.map = map;
}
@Override
public void run() {
for (int i = 0; i < 100; i++) {
synchronized (map) { // 对共享对象map上锁
this.addup();
}
}
}
private void addup() {
if (!map.containsKey(KEY)) {
map.put(KEY, 1);
} else {
map.put(KEY, map.get(KEY) + 1);
}
}
}
此时在调用addup时直接锁定map,由于map是被所有线程共享的,因而达到了让所有线程互斥的目的,线程安全达成。
注意:
synchronized {普通方法}依靠对象锁工作,多线程访问synchronized方法,一旦某个进程抢得锁之后,其他的进程只有排队对待。synchronized void method{}功能上,等效于
void method{
synchronized(this) {
...
}
}
2.synchronized {static方法}此代码块等效于
void method{
synchronized(Obl.class)
}
}
使用该类的类对象的锁定去做线程的共享互斥.
在spring源码中的使用:DefaultListableBeanFactory.java registerBeanDefinition
/** Map of bean definition objects, keyed by bean name */
private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<String, BeanDefinition>(64);
//---------------------------------------------------------------------
// Implementation of BeanDefinitionRegistry interface
//---------------------------------------------------------------------
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
throws BeanDefinitionStoreException {
Assert.hasText(beanName, "Bean name must not be empty");
Assert.notNull(beanDefinition, "BeanDefinition must not be null");
if (beanDefinition instanceof AbstractBeanDefinition) {
try {
((AbstractBeanDefinition) beanDefinition).validate();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Validation of bean definition failed", ex);
}
}
BeanDefinition oldBeanDefinition;
synchronized (this.beanDefinitionMap) {
oldBeanDefinition = this.beanDefinitionMap.get(beanName);
if (oldBeanDefinition != null) {
if (!this.allowBeanDefinitionOverriding) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(),beanName,
"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
"': There is already [" + oldBeanDefinition + "] bound.");
}
else {
if (this.logger.isInfoEnabled()) {
this.logger.info("Overriding bean definition for bean '" + beanName +
"': replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");
}
}
}
else {
this.beanDefinitionNames.add(beanName);
this.frozenBeanDefinitionNames = null;
}
this.beanDefinitionMap.put(beanName, beanDefinition);
}
if (oldBeanDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
}
除去项目中的业务逻辑,
4000
简化后的代码如下:
[java] view
plain copy
public class Test40 {
public static void main(String[] args) throws InterruptedException {
for (int i = 0; i < 10; i++) {
System.out.println(test());
}
}
private static int test() throws InterruptedException {
ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<String, Integer>();
ExecutorService pool = Executors.newCachedThreadPool();
for (int i = 0; i < 8; i++) {
pool.execute(new MyTask(map));
}
pool.shutdown();
pool.awaitTermination(1, TimeUnit.DAYS);
return map.get(MyTask.KEY);
}
}
class MyTask implements Runnable {
public static final String KEY = "key";
private ConcurrentHashMap<String, Integer> map;
public MyTask(ConcurrentHashMap<String, Integer> map) {
this.map = map;
}
@Override
public void run() {
for (int i = 0; i < 100; i++) {
this.addup();
}
}
private void addup() {
if (!map.containsKey(KEY)) {
map.put(KEY, 1);
} else {
map.put(KEY, map.get(KEY) + 1);
}
}
}
测试代码跑了10次,每次都不是800。这就很让人疑惑了,难道ConcurrentHashMap的线程安全性失效了?
查了一些资料后发现,原来ConcurrentHashMap的线程安全指的是,它的每个方法单独调用(即原子操作)都是线程安全的,但是代码总体的互斥性并不受控制。以上面的代码为例,最后一行中的:
[java] view
plain copy
map.put(KEY, map.get(KEY) + 1);
实际上并不是原子操作,它包含了三步:
map.get
加1
map.put
其中第1和第3步,单独来说都是线程安全的,由ConcurrentHashMap保证。但是由于在上面的代码中,map本身是一个共享变量。当线程A执行map.get的时候,其它线程可能正在执行map.put,这样一来当线程A执行到map.put的时候,线程A的值就已经是脏数据了,然后脏数据覆盖了真值,导致线程不安全
简单地说,ConcurrentHashMap的get方法获取到的是此时的真值,但它并不保证当你调用put方法的时候,当时获取到的值仍然是真值
为了使上面的代码变得线程安全,我引入了synchronized关键字来修饰目标方法,如下:
[java] view
plain copy
public class Test40 {
public static void main(String[] args) throws InterruptedException {
for (int i = 0; i < 10; i++) {
System.out.println(test());
}
}
private static int test() throws InterruptedException {
ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<String, Integer>();
ExecutorService pool = Executors.newCachedThreadPool();
for (int i = 0; i < 8; i++) {
pool.execute(new MyTask(map));
}
pool.shutdown();
pool.awaitTermination(1, TimeUnit.DAYS);
return map.get(MyTask.KEY);
}
}
class MyTask implements Runnable {
public static final String KEY = "key";
private ConcurrentHashMap<String, Integer> map;
public MyTask(ConcurrentHashMap<String, Integer> map) {
this.map = map;
}
@Override
public void run() {
for (int i = 0; i < 100; i++) {
this.addup();
}
}
private synchronized void addup() { // 用关键字synchronized修饰addup方法
if (!map.containsKey(KEY)) {
map.put(KEY, 1);
} else {
map.put(KEY, map.get(KEY) + 1);
}
}
}
运行之后仍然是线程不安全的,难道synchronized也失效了?
查阅了synchronized的资料后,原来,不管synchronized是用来修饰方法,还是修饰代码块,其本质都是锁定某一个对象。修饰方法时,锁上的是调用这个方法的对象,即this;修饰代码块时,锁上的是括号里的那个对象
在上面的代码中,很明显就是锁定的MyTask对象本身。但是由于在每一个线程中,MyTask对象都是独立的,这就导致实际上每个线程都对自己的MyTask进行锁定,而并不会干涉其它线程的MyTask对象。换言之,上锁压根没有意义
理解到这点之后,对上面的代码又做了一次修改:
[java] view
plain copy
public class Test40 {
public static void main(String[] args) throws InterruptedException {
for (int i = 0; i < 10; i++) {
System.out.println(test());
}
}
private static int test() throws InterruptedException {
ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<String, Integer>();
ExecutorService pool = Executors.newCachedThreadPool();
for (int i = 0; i < 8; i++) {
pool.execute(new MyTask(map));
}
pool.shutdown();
pool.awaitTermination(1, TimeUnit.DAYS);
return map.get(MyTask.KEY);
}
}
class MyTask implements Runnable {
public static final String KEY = "key";
private ConcurrentHashMap<String, Integer> map;
public MyTask(ConcurrentHashMap<String, Integer> map) {
this.map = map;
}
@Override
public void run() {
for (int i = 0; i < 100; i++) {
synchronized (map) { // 对共享对象map上锁
this.addup();
}
}
}
private void addup() {
if (!map.containsKey(KEY)) {
map.put(KEY, 1);
} else {
map.put(KEY, map.get(KEY) + 1);
}
}
}
此时在调用addup时直接锁定map,由于map是被所有线程共享的,因而达到了让所有线程互斥的目的,线程安全达成。
注意:
synchronized {普通方法}依靠对象锁工作,多线程访问synchronized方法,一旦某个进程抢得锁之后,其他的进程只有排队对待。synchronized void method{}功能上,等效于
void method{
synchronized(this) {
...
}
}
2.synchronized {static方法}此代码块等效于
void method{
synchronized(Obl.class)
}
}
使用该类的类对象的锁定去做线程的共享互斥.
在spring源码中的使用:DefaultListableBeanFactory.java registerBeanDefinition
/** Map of bean definition objects, keyed by bean name */
private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<String, BeanDefinition>(64);
//---------------------------------------------------------------------
// Implementation of BeanDefinitionRegistry interface
//---------------------------------------------------------------------
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
throws BeanDefinitionStoreException {
Assert.hasText(beanName, "Bean name must not be empty");
Assert.notNull(beanDefinition, "BeanDefinition must not be null");
if (beanDefinition instanceof AbstractBeanDefinition) {
try {
((AbstractBeanDefinition) beanDefinition).validate();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Validation of bean definition failed", ex);
}
}
BeanDefinition oldBeanDefinition;
synchronized (this.beanDefinitionMap) {
oldBeanDefinition = this.beanDefinitionMap.get(beanName);
if (oldBeanDefinition != null) {
if (!this.allowBeanDefinitionOverriding) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(),beanName,
"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
"': There is already [" + oldBeanDefinition + "] bound.");
}
else {
if (this.logger.isInfoEnabled()) {
this.logger.info("Overriding bean definition for bean '" + beanName +
"': replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");
}
}
}
else {
this.beanDefinitionNames.add(beanName);
this.frozenBeanDefinitionNames = null;
}
this.beanDefinitionMap.put(beanName, beanDefinition);
}
if (oldBeanDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
}
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