xodus,JeTeWork XODUS是JTeWord YouTrand和JETHealthHub使用的一个Java事务无模式...

# <img src="https://raw.githubusercontent.com/wiki/jetbrains/xodus/xodus.png" width=160> The **Entity Stores** layer is designed to access data as [entities](https://github.com/JetBrains/xodus/blob/master/openAPI/src/main/java/jetbrains/exodus/entitystore/Entity.java) with attributes and links. Use a [transaction](https://github.com/JetBrains/xodus/blob/master/openAPI/src/main/java/jetbrains/exodus/entitystore/StoreTransaction.java) to create, modify, read and query data. Transactions are quite similar to [those](https://github.com/JetBrains/xodus/blob/master/openAPI/src/main/java/jetbrains/exodus/env/Transaction.java) on the **Environments** layer, though the **Entity Store** API is much richer in terms of querying data. The API and the implementation live in the _jetbrains.exodus.entitystore_ package. [PersistentEntityStore](https://github.com/JetBrains/xodus/wiki/Entity-Stores#persistententitystore) <br>[Transactions](https://github.com/JetBrains/xodus/wiki/Entity-Stores#transactions) <br>[Entities](https://github.com/JetBrains/xodus/wiki/Entity-Stores#entities) <br>[Properties](https://github.com/JetBrains/xodus/wiki/Entity-Stores#properties) <br>[Links](https://github.com/JetBrains/xodus/wiki/Entity-Stores#links) <br>[Blobs](https://github.com/JetBrains/xodus/wiki/Entity-Stores#blobs) <br>[Queries](https://github.com/JetBrains/xodus/wiki/Entity-Stores#queries) <br>&nbsp;&nbsp;&nbsp;&nbsp;[Iterating over All Entities of Specified Type](https://github.com/JetBrains/xodus/wiki/Entity-Stores#iterating-over-all-entities-of-specified-type) <br>&nbsp;&nbsp;&nbsp;&nbsp;[EntityIterable](https://github.com/JetBrains/xodus/wiki/Entity-Stores#entityiterable) <br>&nbsp;&nbsp;&nbsp;&nbsp;[EntityIterator](https://github.com/JetBrains/xodus/wiki/Entity-Stores#entityiterator) <br>&nbsp;&nbsp;&nbsp;&nbsp;[Searching by Property Value](https://github.com/JetBrains/xodus/wiki/Entity-Stores#searching-by-property-value) <br>&nbsp;&nbsp;&nbsp;&nbsp;[Searching in Range of Property Values](https://github.com/JetBrains/xodus/wiki/Entity-Stores#searching-in-range-of-property-values) <br>&nbsp;&nbsp;&nbsp;&nbsp;[Traversing Links](https://github.com/JetBrains/xodus/wiki/Entity-Stores#traversing-links) <br>&nbsp;&nbsp;&nbsp;&nbsp;[SelectDistinct and SelectManyDistinct](https://github.com/JetBrains/xodus/wiki/Entity-Stores#selectdistinct-and-selectmanydistinct) <br>&nbsp;&nbsp;&nbsp;&nbsp;[Binary Operations](https://github.com/JetBrains/xodus/wiki/Entity-Stores#binary-operations) <br>&nbsp;&nbsp;&nbsp;&nbsp;[Searching for Entities Having Property, Link, Blob](https://github.com/JetBrains/xodus/wiki/Entity-Stores#searching-for-entities-having-property-link-blob) <br>&nbsp;&nbsp;&nbsp;&nbsp;[Sorting](https://github.com/JetBrains/xodus/wiki/Entity-Stores#sorting) <br>&nbsp;&nbsp;&nbsp;&nbsp;[Other Goodies](https://github.com/JetBrains/xodus/wiki/Entity-Stores#other-goodies) <br>[Sequences](https://github.com/JetBrains/xodus/wiki/Entity-Stores#sequences) ## PersistentEntityStore To open or create an entity store, create an instance of [PersistentEntityStore](https://github.com/JetBrains/xodus/blob/master/openAPI/src/main/java/jetbrains/exodus/entitystore/PersistentEntityStore.java) with the help of the [PersistentEntityStores](https://github.com/JetBrains/xodus/blob/master/entity-store/src/main/java/jetbrains/exodus/entitystore/PersistentEntityStores.java) utility class: ```java PersistentEntityStore entityStore = PersistentEntityStores.newInstance("/home/me/.myAppData"); ``` `PersistentEntityStore` works over `Environment`, so the method that is shown above implicitly creates an `Environment` with the same location. Each `PersistentEntityStore` has a name. You can create several entity stores with different names over an `Environment`. If you don't specify a name, the [default name](https://github.com/JetBrains/xodus/blob/master/entity-store/src/main/java/jetbrains/exodus/entitystore/PersistentEntityStores.java#L30) is used. `PersistentEntityStore` has different methods to create an instance of a `PersistentEntityStore`. In addition to the underlying `Environment`, you can specify the [BlobValut](https://github.com/JetBrains/xodus/blob/master/openAPI/src/main/java/jetbrains/exodus/entitystore/BlobVault.java) and [PersistentEntityStoreConfig](https://github.com/JetBrains/xodus/blob/master/openAPI/src/main/java/jetbrains/exodus/entitystore/PersistentEntityStoreConfig.java). `BlobVault` is a base class that describes an interface to binary large objects (BLOBs) that are used internally by the implementation of a `PersistentEntityStore`. If you don't specify a `BlobVault` when you create a `PersistentEntityStore`, an instance of the [FileSystemBlobVault](https://github.com/JetBrains/xodus/blob/master/openAPI/src/main/java/jetbrains/exodus/entitystore/FileSystemBlobVault.java) class is used. If you don't specify the `PersistentEntityStoreConfig` when you create a `PersistentEntityStore`, [PersistentEntityStoreConfig.DEFAULT](https://github.com/JetBrains/xodus/blob/master/openAPI/src/main/java/jetbrains/exodus/entitystore/PersistentEntityStoreConfig.java#L58) is used. Like [ContextualEnvironment](https://github.com/JetBrains/xodus/blob/master/openAPI/src/main/java/jetbrains/exodus/env/ContextualEnvironment.java), `PersistentEntityStore` is always aware of the transaction that is started in the current thread. The [getCurrentTransaction()](https://github.com/JetBrains/xodus/blob/master/openAPI/src/main/java/jetbrains/exodus/entitystore/EntityStore.java#L84) method returns the transaction that is started in the current thread or null if there is no such transaction. When you are finished working with a `PersistentEntityStore`, call the `close()` method. ## Transactions [Entity store transactions](https://github.com/JetBrains/xodus/blob/master/openAPI/src/main/java/jetbrains/exodus/entitystore/StoreTransaction.java) are quite similar to the `Environment` layer transactions. To manually start a transaction, use [beginTransaction()](https://github.com/JetBrains/xodus/blob/master/openAPI/src/main/java/jetbrains/exodus/entitystore/EntityStore.java#L56): ```java final StoreTransaction txn = store.beginTransaction(); ``` or [beginReadonlyTransaction()](https://github.com/JetBrains/xodus/blob/master/openAPI/src/main/java/jetbrains/exodus/entitystore/EntityStore.java#L67): ```java final StoreTransaction txn = store.beginReadonlyTransaction(); ``` An attempt to modify data in a read-only transaction fails with a `ReadonlyTransactionException`. Any transaction should be finished, meaning that it is either aborted or committed. The transaction can also be flushed or reverted. The methods `commit()` and `flush()` return `true` if they succeed. If any method returns `false`, a database version mismatch has occurred. In this case, there are two possibilities: to abort the transaction and finish or revert the transaction and continue. An unsuccessful flush implicitly reverts the transaction and moves it to the latest (newest) database snapshot, so database operations can be repeated against it: ```java StoreTransaction txn = beginTransaction(); try { do { // do something // if txn has already been aborted in user code if (txn != getCurrentTransaction()) { txn = null; break; } } while (!txn.flush()); } finally { // if txn has not already been aborted in execute() if (txn != null) { txn.abort(); } } ``` If you don't care for such spinning and don't want to control the results of `flush()` and `commit()`, you can use the [executeInTransaction()](https://github.com/JetBrains/xodus/blob/master/openAPI/src/main/java/jetbrains/exodus/entitystore/PersistentEntityStore.java#L63), [executeInExclusiveTransaction()](https://github.com/JetBrains/xodus/blob/master/openAPI/src/main/java/jetbrains/exodus/entitystore/PersistentEntityStore.java#L73), [executeInReadonlyTransaction()](https://github.com/JetBrains/xodus/blob/ma