我有一个单例包装器类，为我的应用程序抽象了Elasticsearch API。
public class ElasticSearchClient {
private static volatile ElasticSearchClient elasticSearchClientInstance;
private static final Object lock = new Object();
private static elasticConfig ;
/*
** Private constructor to make this class singleton
*/
private ElasticSearchClient() {
}
/*
** This method does a lazy initialization and returns the singleton instance of ElasticSearchClient
*/
public static ElasticSearchClient getInstance() {
ElasticSearchClient elasticSearchClientInstanceToReturn = elasticSearchClientInstance;
if (elasticSearchClientInstanceToReturn == null) {
synchronized(lock) {
elasticSearchClientInstanceToReturn = elasticSearchClientInstance;
if (elasticSearchClientInstanceToReturn == null) {
// While this thread was waiting for the lock, another thread may have instantiated the clinet.
elasticSearchClientInstanceToReturn = new ElasticSearchClient();
elasticSearchClientInstance = elasticSearchClientInstanceToReturn;
}
}
}
return elasticSearchClientInstanceToReturn;
}
/*
** This method creates a new elastic index with the name as the paramater, if if does not already exists.
* Returns true if the index creation is successful, false otherwise.
*/
public boolean createElasticIndex(String index) {
if (checkIfElasticSearchIndexExists(index)) {
LOG.error("Cannot recreate already existing index: " + index);
return false;
}
if (elasticConfig == null || elasticConfig.equals(BatchConstants.EMPTY_STRING)) {
loadElasticConfigFromFile(ELASTIC_CONFIG_FILE_NAME);
}
if (elasticConfig != null && !elasticConfig.equals("")) {
try {
HttpURLConnection elasticSearchHttpURLConnection = performHttpRequest(
ELASTIC_SEARCH_URL + "/" + index,
"PUT",
elasticConfig,
"Create index: " + index
);
return elasticSearchHttpURLConnection != null &&
elasticSearchHttpURLConnection.getResponseCode() == HttpURLConnection.HTTP_OK;
} catch (Exception e) {
LOG.error("Unable to access Elastic Search API. Following exception occurred:\n" + e.getMessage());
}
} else {
LOG.error("Found empty config file");
}
return false;
}
private void loadElasticConfigFromFile(String filename) {
try {
Object obj = jsonParser.parse(new FileReader(filename);
JSONObject jsonObject = (JSONObject) obj;
LOG.info("Successfully parsed elastic config file: "+ filename);
elasticConfig = jsonObject.toString();
return;
} catch (Exception e) {
LOG.error("Cannot read elastic config from " + filename + "\n" + e.getMessage());
elasticConfig = "";
}
}
}
我有多个使用ElasticSearchClient的线程，如下所述
Thread1
ElasticSearchClient elasticSearchClient = ElasticSearchClient.getInstance()
elasticSearchClient.createElasticIndex("firstindex");
Thread2
ElasticSearchClient elasticSearchClient = ElasticSearchClient.getInstance()
elasticSearchClient.createElasticIndex("secondindex");
Thread3...
按照我的说法，Singleton类是线程安全的，但是我不确定如果多个线程开始执行与Singleton类相同的方法会发生什么。这有副作用吗？
注意：我知道上面的单例类不是反射和序列化安全的。
最佳答案
在您的特定实现中
if (checkIfElasticSearchIndexExists(index)) { //NOT THREAD SAFE
LOG.error("Cannot recreate already existing index: " + index);
return false;
}
if (elasticConfig == null || elasticConfig.equals(BatchConstants.EMPTY_STRING)) { //NOT THREAD SAFE
loadElasticConfigFromFile(ELASTIC_CONFIG_FILE_NAME);
}
if (elasticConfig != null && !elasticConfig.equals("")) { //NOT THREAD SAFE
有3分可能会导致比赛状况。
所以本身
单例类公共方法是否应该同步？
没有这样的规则-如果那些规则是线程安全的，则不需要其他同步。在您的情况下，这些线程不是线程安全的，因此您必须使它们脱险。制造
public synchronized boolean createElasticIndex
如果您对例如并发写入单个索引比较满意，那就不要-这是一个ElasticSearch任务，可以正确处理并发写入（相信我，ES会顺利处理）
什么不是线程安全的（指出3个地方）？具有并发的T1和T2：
checkIfElasticSearchIndexExists(index)如果T1和T2将使用相同的索引名称，则两者都将通过此检查（我仅假设这是一个rest调用-甚至更糟）elasticConfig == null || elasticConfig.equals(BatchConstants.EMPTY_STRING)在第一个矮点T1和T2都将通过此测试，并且都将从文件中加载配置-可能不会产生影响，但仍然是赛车情况if (elasticConfig != null && !elasticConfig.equals(""))与2 +相同的情况（由于内存模型），如果elasticConfig不是volatile，则在loadElasticConfigFromFile将其完全初始化之前，可以将其读取为“ not null”。2和3可以通过双重检查锁定来固定（就像您在
getInstance()中所做的那样，或者我宁愿将其移至实例初始化块中-构造函数将是我认为的最佳选择。为了更好地了解该现象，您可以检查why a==1 && a==2 can evaluate to true
但是，由于调用和响应之间存在延迟，因此1是一个更大的问题，您得到了一个宽阔的窗口，其中2个线程可以查询相同的索引并获得完全相同的响应-该索引不存在并尝试创建一个。