## Chapter 2. Creating and Destroying Objects(創建和銷毀對象)
### Item 9: Prefer try-with-resources to try-finally(使用 try-with-resources 優于 try-finally)
The Java libraries include many resources that must be closed manually by invoking a close method. Examples include InputStream,OutputStream, and java.sql.Connection. Closing resources is often overlooked by clients, with predictably dire performance consequences. While many of these resources use finalizers as a safety net, finalizers don’t work very well (Item 8).
Java 庫包含許多必須通過調用 close 方法手動關閉的資源。常見的有 InputStream、OutputStream 和 java.sql.Connection。關閉資源常常會被客戶端忽略,這會導致可怕的性能后果。雖然這些資源中的許多都使用終結器作為安全網,但終結器并不能很好地工作([Item-8](/Chapter-2/Chapter-2-Item-8-Avoid-finalizers-and-cleaners.md))。
Historically, a try-finally statement was the best way to guarantee that a resource would be closed properly, even in the face of an exception or return:
從歷史上看,try-finally 語句是確保正確關閉資源的最佳方法,即使在出現異常或返回時也是如此:
```
// try-finally - No longer the best way to close resources!
static String firstLineOfFile(String path) throws IOException {
BufferedReader br = new BufferedReader(new FileReader(path));
try {
return br.readLine();
} finally {
br.close();
}
}
```
This may not look bad, but it gets worse when you add a second resource:
這可能看起來不壞,但添加第二個資源時,情況會變得更糟:
```
// try-finally is ugly when used with more than one resource!
static void copy(String src, String dst) throws IOException {
InputStream in = new FileInputStream(src);
try {
OutputStream out = new FileOutputStream(dst);
try {
byte[] buf = new byte[BUFFER_SIZE];
int n;
while ((n = in.read(buf)) >= 0)
out.write(buf, 0, n);
} finally {
out.close();
}
}
finally {
in.close();
}
}
```
It may be hard to believe, but even good programmers got this wrong most of the time. For starters, I got it wrong on page 88 of Java Puzzlers [Bloch05], and no one noticed for years. In fact, two-thirds of the uses of the close method in the Java libraries were wrong in 2007.
這可能難以置信。在大多數情況下,即使是優秀的程序員也會犯這種錯誤。首先,我在 Java Puzzlers [Bloch05]的 88 頁上做錯了,多年來沒有人注意到。事實上,2007 年發布的 Java 庫中三分之二的 close 方法使用都是錯誤的。
**譯注:《Java Puzzlers》的中文譯本為《Java 解惑》**
Even the correct code for closing resources with try-finally statements,as illustrated in the previous two code examples, has a subtle deficiency. The code in both the try block and the finally block is capable of throwing exceptions. For example, in the firstLineOfFile method, the call to readLine could throw an exception due to a failure in the underlying physical device, and the call to close could then fail for the same reason. Under these circumstances, the second exception completely obliterates the first one. There is no record of the first exception in the exception stack trace, which can greatly complicate debugging in real systems—usually it’s the first exception that you want to see in order to diagnose the problem. While it is possible to write code to suppress the second exception in favor of the first, virtually no one did because it’s just too verbose.
使用 try-finally 語句關閉資源的正確代碼(如前兩個代碼示例所示)也有一個細微的缺陷。try 塊和 finally 塊中的代碼都能夠拋出異常。例如,在 firstLineOfFile 方法中,由于底層物理設備發生故障,對 readLine 的調用可能會拋出異常,而關閉的調用也可能出于同樣的原因而失敗。在這種情況下,第二個異常將完全覆蓋第一個異常。異常堆棧跟蹤中沒有第一個異常的記錄,這可能會使實際系統中的調試變得非常復雜(而這可能是希望出現的第一個異常,以便診斷問題)。雖然可以通過編寫代碼來抑制第二個異常而支持第一個異常,但實際上沒有人這樣做,因為它太過冗長。
All of these problems were solved in one fell swoop when Java 7 introduced the try-with-resources statement [JLS, 14.20.3]. To be usable with this construct, a resource must implement the AutoCloseable interface, which consists of a single void-returning close method. Many classes and interfaces in the Java libraries and in third-party libraries now implement or extend AutoCloseable. If you write a class that represents a resource that must be closed, your class should implement AutoCloseable too.
當 Java 7 引入 try-with-resources 語句 [JLS, 14.20.3]時,所有這些問題都一次性解決了。要使用這個結構,資源必須實現 AutoCloseable 接口,它由一個單獨的 void-return close 方法組成。Java 庫和第三方庫中的許多類和接口現在都實現或擴展了 AutoCloseable。如果你編寫的類存在必須關閉的資源,那么也應該實現 AutoCloseable。
Here’s how our first example looks using try-with-resources:
下面是使用 try-with-resources 的第一個示例:
```
// try-with-resources - the the best way to close resources!
static String firstLineOfFile(String path) throws IOException {
try (BufferedReader br = new BufferedReader(new FileReader(path))) {
return br.readLine();
}
}
```
And here’s how our second example looks using try-with-resources:
下面是使用 try-with-resources 的第二個示例:
```
// try-with-resources on multiple resources - short and sweet
static void copy(String src, String dst) throws IOException {
try (InputStream in = new FileInputStream(src);OutputStream out = new FileOutputStream(dst)) {
byte[] buf = new byte[BUFFER_SIZE];
int n;
while ((n = in.read(buf)) >= 0)
out.write(buf, 0, n);
}
}
```
originals, but they provide far better diagnostics. Consider the firstLineOfFile method. If exceptions are thrown by both the readLine call and the (invisible) close, the latter exception is suppressed in favor of the former. In fact, multiple exceptions may be suppressed in order to preserve the exception that you actually want to see. These suppressed exceptions are not merely discarded; they are printed in the stack trace with a notation saying that they were suppressed. You can also access them programmatically with the getSuppressed method, which was added to Throwable in Java 7.
和使用 try-finally 的原版代碼相比,try-with-resources 為開發者提供了更好的診斷方式。考慮 firstLineOfFile 方法。如果異常是由 readLine 調用和不可見的 close 拋出的,則后一個異常將被抑制,以支持前一個異常。實際上,還可能會抑制多個異常,以保留實際希望看到的異常。這些被抑制的異常不會僅僅被拋棄;它們會被打印在堆棧跟蹤中,并標記它們被抑制。可以通過編程方式使用 getSuppressed 方法訪問到它們,該方法是在 Java 7 中添加到 Throwable 中的。
You can put catch clauses on try-with-resources statements, just as you can on regular try-finally statements. This allows you to handle exceptions without sullying your code with another layer of nesting. As a slightly contrived example, here’s a version our firstLineOfFile method that does not throw exceptions, but takes a default value to return if it can’t open the file or read from it:
可以在帶有資源的 try-with-resources 語句中放置 catch 子句,就像在常規的 try-finally 語句上一樣。這允許處理異常時不必用另一層嵌套來影響代碼。作為一個特指的示例,下面是我們的 firstLineOfFile 方法的一個版本,它不拋出異常,但如果無法打開文件或從中讀取文件,則返回一個默認值:
```
// try-with-resources with a catch clause
static String firstLineOfFile(String path, String defaultVal) {
try (BufferedReader br = new BufferedReader(new FileReader(path))) {
return br.readLine();
} catch (IOException e) {
return defaultVal;
}
}
```
The lesson is clear: Always use try-with-resources in preference to tryfinally when working with resources that must be closed. The resulting code is shorter and clearer, and the exceptions that it generates are more useful. The try-with-resources statement makes it easy to write correct code using resources that must be closed, which was practically impossible using tryfinally.
教訓很清楚:在使用必須關閉的資源時,總是優先使用 try-with-resources,而不是 try-finally。前者的代碼更短、更清晰,生成的異常更有用。使用 try-with-resources 語句可以很容易地為必須關閉的資源編寫正確的代碼,而使用 try-finally 幾乎是不可能的。
---
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- Chapter 2. Creating and Destroying Objects(創建和銷毀對象)
- Item 1: Consider static factory methods instead of constructors(考慮以靜態工廠方法代替構造函數)
- Item 2: Consider a builder when faced with many constructor parameters(在面對多個構造函數參數時,請考慮構建器)
- Item 3: Enforce the singleton property with a private constructor or an enum type(使用私有構造函數或枚舉類型實施單例屬性)
- Item 4: Enforce noninstantiability with a private constructor(用私有構造函數實施不可實例化)
- Item 5: Prefer dependency injection to hardwiring resources(依賴注入優于硬連接資源)
- Item 6: Avoid creating unnecessary objects(避免創建不必要的對象)
- Item 7: Eliminate obsolete object references(排除過時的對象引用)
- Item 8: Avoid finalizers and cleaners(避免使用終結器和清除器)
- Item 9: Prefer try with resources to try finally(使用 try-with-resources 優于 try-finally)
- Chapter 3. Methods Common to All Objects(對象的通用方法)
- Item 10: Obey the general contract when overriding equals(覆蓋 equals 方法時應遵守的約定)
- Item 11: Always override hashCode when you override equals(當覆蓋 equals 方法時,總要覆蓋 hashCode 方法)
- Item 12: Always override toString(始終覆蓋 toString 方法)
- Item 13: Override clone judiciously(明智地覆蓋 clone 方法)
- Item 14: Consider implementing Comparable(考慮實現 Comparable 接口)
- Chapter 4. Classes and Interfaces(類和接口)
- Item 15: Minimize the accessibility of classes and members(盡量減少類和成員的可訪問性)
- Item 16: In public classes use accessor methods not public fields(在公共類中,使用訪問器方法,而不是公共字段)
- Item 17: Minimize mutability(減少可變性)
- Item 18: Favor composition over inheritance(優先選擇復合而不是繼承)
- Item 19: Design and document for inheritance or else prohibit it(繼承要設計良好并且具有文檔,否則禁止使用)
- Item 20: Prefer interfaces to abstract classes(接口優于抽象類)
- Item 21: Design interfaces for posterity(為后代設計接口)
- Item 22: Use interfaces only to define types(接口只用于定義類型)
- Item 23: Prefer class hierarchies to tagged classes(類層次結構優于帶標簽的類)
- Item 24: Favor static member classes over nonstatic(靜態成員類優于非靜態成員類)
- Item 25: Limit source files to a single top level class(源文件僅限有單個頂層類)
- Chapter 5. Generics(泛型)
- Item 26: Do not use raw types(不要使用原始類型)
- Item 27: Eliminate unchecked warnings(消除 unchecked 警告)
- Item 28: Prefer lists to arrays(list 優于數組)
- Item 29: Favor generic types(優先使用泛型)
- Item 30: Favor generic methods(優先使用泛型方法)
- Item 31: Use bounded wildcards to increase API flexibility(使用有界通配符增加 API 的靈活性)
- Item 32: Combine generics and varargs judiciously(明智地合用泛型和可變參數)
- Item 33: Consider typesafe heterogeneous containers(考慮類型安全的異構容器)
- Chapter 6. Enums and Annotations(枚舉和注解)
- Item 34: Use enums instead of int constants(用枚舉類型代替 int 常量)
- Item 35: Use instance fields instead of ordinals(使用實例字段替代序數)
- Item 36: Use EnumSet instead of bit fields(用 EnumSet 替代位字段)
- Item 37: Use EnumMap instead of ordinal indexing(使用 EnumMap 替換序數索引)
- Item 38: Emulate extensible enums with interfaces(使用接口模擬可擴展枚舉)
- Item 39: Prefer annotations to naming patterns(注解優于命名模式)
- Item 40: Consistently use the Override annotation(堅持使用 @Override 注解)
- Item 41: Use marker interfaces to define types(使用標記接口定義類型)
- Chapter 7. Lambdas and Streams(λ 表達式和流)
- Item 42: Prefer lambdas to anonymous classes(λ 表達式優于匿名類)
- Item 43: Prefer method references to lambdas(方法引用優于 λ 表達式)
- Item 44: Favor the use of standard functional interfaces(優先使用標準函數式接口)
- Item 45: Use streams judiciously(明智地使用流)
- Item 46: Prefer side effect free functions in streams(在流中使用無副作用的函數)
- Item 47: Prefer Collection to Stream as a return type(優先選擇 Collection 而不是流作為返回類型)
- Item 48: Use caution when making streams parallel(謹慎使用并行流)
- Chapter 8. Methods(方法)
- Item 49: Check parameters for validity(檢查參數的有效性)
- Item 50: Make defensive copies when needed(在需要時制作防御性副本)
- Item 51: Design method signatures carefully(仔細設計方法簽名)
- Item 52: Use overloading judiciously(明智地使用重載)
- Item 53: Use varargs judiciously(明智地使用可變參數)
- Item 54: Return empty collections or arrays, not nulls(返回空集合或數組,而不是 null)
- Item 55: Return optionals judiciously(明智地的返回 Optional)
- Item 56: Write doc comments for all exposed API elements(為所有公開的 API 元素編寫文檔注釋)
- Chapter 9. General Programming(通用程序設計)
- Item 57: Minimize the scope of local variables(將局部變量的作用域最小化)
- Item 58: Prefer for-each loops to traditional for loops(for-each 循環優于傳統的 for 循環)
- Item 59: Know and use the libraries(了解并使用庫)
- Item 60: Avoid float and double if exact answers are required(若需要精確答案就應避免使用 float 和 double 類型)
- Item 61: Prefer primitive types to boxed primitives(基本數據類型優于包裝類)
- Item 62: Avoid strings where other types are more appropriate(其他類型更合適時應避免使用字符串)
- Item 63: Beware the performance of string concatenation(當心字符串連接引起的性能問題)
- Item 64: Refer to objects by their interfaces(通過接口引用對象)
- Item 65: Prefer interfaces to reflection(接口優于反射)
- Item 66: Use native methods judiciously(明智地使用本地方法)
- Item 67: Optimize judiciously(明智地進行優化)
- Item 68: Adhere to generally accepted naming conventions(遵守被廣泛認可的命名約定)
- Chapter 10. Exceptions(異常)
- Item 69: Use exceptions only for exceptional conditions(僅在確有異常條件下使用異常)
- Item 70: Use checked exceptions for recoverable conditions and runtime exceptions for programming errors(對可恢復情況使用 checked 異常,對編程錯誤使用運行時異常)
- Item 71: Avoid unnecessary use of checked exceptions(避免不必要地使用 checked 異常)
- Item 72: Favor the use of standard exceptions(鼓勵復用標準異常)
- Item 73: Throw exceptions appropriate to the abstraction(拋出能用抽象解釋的異常)
- Item 74: Document all exceptions thrown by each method(為每個方法記錄會拋出的所有異常)
- Item 75: Include failure capture information in detail messages(異常詳細消息中應包含捕獲失敗的信息)
- Item 76: Strive for failure atomicity(盡力保證故障原子性)
- Item 77: Don’t ignore exceptions(不要忽略異常)
- Chapter 11. Concurrency(并發)
- Item 78: Synchronize access to shared mutable data(對共享可變數據的同步訪問)
- Item 79: Avoid excessive synchronization(避免過度同步)
- Item 80: Prefer executors, tasks, and streams to threads(Executor、task、流優于直接使用線程)
- Item 81: Prefer concurrency utilities to wait and notify(并發實用工具優于 wait 和 notify)
- Item 82: Document thread safety(文檔應包含線程安全屬性)
- Item 83: Use lazy initialization judiciously(明智地使用延遲初始化)
- Item 84: Don’t depend on the thread scheduler(不要依賴線程調度器)
- Chapter 12. Serialization(序列化)
- Item 85: Prefer alternatives to Java serialization(優先選擇 Java 序列化的替代方案)
- Item 86: Implement Serializable with great caution(非常謹慎地實現 Serializable)
- Item 87: Consider using a custom serialized form(考慮使用自定義序列化形式)
- Item 88: Write readObject methods defensively(防御性地編寫 readObject 方法)
- Item 89: For instance control, prefer enum types to readResolve(對于實例控制,枚舉類型優于 readResolve)
- Item 90: Consider serialization proxies instead of serialized instances(考慮以序列化代理代替序列化實例)