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使用try-with-resources语句替代try-finally语句
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尉勇强2
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@ -32,3 +32,4 @@ finalizer机制有一个严重的安全问题：它们会打开你的类来进
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Cleaner机制的规范说：“`System.exit`方法期间的清理行为是特定于实现的。 不保证清理行为是否被调用。”虽然规范没有说明，但对于正常的程序退出也是如此。 在我的机器上，将`System.gc()`方法添加到`Teenager`类的`main`方法足以让程序退出之前打印`Cleaning room`，但不能保证在你的机器上会看到相同的行为。
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总之，除了作为一个安全网或者终止非关键的本地资源，不要使用Cleaner机制，或者是在Java 9发布之前的finalizers机制。即使是这样，也要当心不确定性和性能影响。
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94
ch02创建和销毁对象/09.使用try-with-resources语句替代try-finally语句.md
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ch02创建和销毁对象/09.使用try-with-resources语句替代try-finally语句.md
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## 使用try-with-resources语句替代try-finally语句
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### 示例
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**示例代码**：[Item09Example01.java](CreatingAndDestroyingObjects/src/main/java/com/jueee/item09/Item09Example01.java)
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### 说明
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从以往来看，try-finally语句是保证资源正确关闭的最佳方式，即使是在程序抛出异常或返回的情况下：
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```java
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// try-finally - No longer the best way to close resources!
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static String firstLineOfFile(String path) throws IOException {
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BufferedReader br = new BufferedReader(new FileReader(path));
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try {
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return br.readLine();
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} finally {
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br.close();
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}
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}
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```
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这可能看起来并不坏，但是当添加第二个资源时，情况会变得更糟：
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```java
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// try-finally is ugly when used with more than one resource!
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static void copy(String src, String dst) throws IOException {
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InputStream in = new FileInputStream(src);
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try {
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OutputStream out = new FileOutputStream(dst);
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try {
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byte[] buf = new byte[BUFFER_SIZE];
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int n;
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while ((n = in.read(buf)) >= 0)
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out.write(buf, 0, n);
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} finally {
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out.close();
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}
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} finally {
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in.close();
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}
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}
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```
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即使是用try-finally语句关闭资源的正确代码，如前面两个代码示例所示，也有一个微妙的缺陷。 try-with-resources块和finally块中的代码都可以抛出异常。 例如，在`firstLineOfFile`方法中，由于底层物理设备发生故障，对`readLine`方法的调用可能会引发异常，并且由于相同的原因，调用`close`方法可能会失败。 在这种情况下，第二个异常完全冲掉了第一个异常。
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当Java 7引入了try-with-resources语句时，所有这些问题一下子都得到了解决。
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要使用这个构造，资源必须实现 `AutoCloseable`接口，该接口由一个返回为`void`的`close`组成。Java类库和第三方类库中的许多类和接口现在都实现或继承了`AutoCloseable`接口。如果你编写的类表示必须关闭的资源，那么这个类也应该实现`AutoCloseable`接口。
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以下是我们的第一个使用try-with-resources的示例：
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```java
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// try-with-resources - the the best way to close resources!
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static String firstLineOfFile(String path) throws IOException {
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try (BufferedReader br = new BufferedReader(
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new FileReader(path))) {
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return br.readLine();
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}
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}
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```
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以下是我们的第二个使用try-with-resources的示例：
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```java
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// try-with-resources on multiple resources - short and sweet
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static void copy(String src, String dst) throws IOException {
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try (InputStream in = new FileInputStream(src);
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OutputStream out = new FileOutputStream(dst)) {
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byte[] buf = new byte[BUFFER_SIZE];
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int n;
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while ((n = in.read(buf)) >= 0)
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out.write(buf, 0, n);
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}
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}
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```
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不仅 try-with-resources版本比原始版本更精简，更好的可读性，而且它们提供了更好的诊断。
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可以在 try-with-resources语句中添加catch子句，就像在常规的try-finally语句中一样。这允许你处理异常，而不会在另一层嵌套中污染代码。作为一个稍微有些做作的例子，这里有一个版本的`firstLineOfFile`方法，它不会抛出异常，但是如果它不能打开或读取文件，则返回默认值：
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```java
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// try-with-resources with a catch clause
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static String firstLineOfFile(String path, String defaultVal) {
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try (BufferedReader br = new BufferedReader(
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new FileReader(path))) {
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return br.readLine();
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} catch (IOException e) {
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return defaultVal;
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}
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}
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```
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结论明确：在处理必须关闭的资源时，使用try-with-resources语句替代try-finally语句。 生成的代码更简洁，更清晰，并且生成的异常更有用。 try-with-resources语句在编写必须关闭资源的代码时会更容易，也不会出错，而使用try-finally语句实际上是不可能的。
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@ -21,7 +21,7 @@
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<scope>test</scope>
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</dependency>
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<!-- https://projectlombok.org/changelog -->
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<!-- v1.18.4:PLATFORM: Many improvements for lombok's JDK10/11 support. -->
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<!-- v1.18.4: PLATFORM: Many improvements for lombok's JDK10/11 support. -->
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<dependency>
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<groupId>org.projectlombok</groupId>
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<artifactId>lombok</artifactId>
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@ -0,0 +1,70 @@
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package com.jueee.item09;
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import java.io.BufferedReader;
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import java.io.FileInputStream;
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import java.io.FileOutputStream;
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import java.io.FileReader;
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import java.io.IOException;
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import java.io.InputStream;
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import java.io.OutputStream;
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public class Item09Example01 {
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private static final int BUFFER_SIZE = 10000;
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// try-finally - 不再是关闭资源的最佳方法！
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static String firstLineOfFile(String path) throws IOException {
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BufferedReader br = new BufferedReader(new FileReader(path));
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try {
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return br.readLine();
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} finally {
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br.close();
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}
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}
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// try-finally 当添加第二个资源时，情况会变得更糟
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static void copy(String src, String dst) throws IOException {
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InputStream in = new FileInputStream(src);
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try {
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OutputStream out = new FileOutputStream(dst);
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try {
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byte[] buf = new byte[BUFFER_SIZE];
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int n;
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while ((n = in.read(buf)) >= 0)
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out.write(buf, 0, n);
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} finally {
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out.close();
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}
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} finally {
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in.close();
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}
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}
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// try-with-resources - 关闭资源的最佳方法！
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static String firstLineOfFileGood(String path) throws IOException {
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try (BufferedReader br = new BufferedReader(new FileReader(path))) {
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return br.readLine();
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}
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}
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// try-with-resources on multiple resources - 关闭资源的最佳方法！
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static void copyGood(String src, String dst) throws IOException {
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try (InputStream in = new FileInputStream(src); OutputStream out = new FileOutputStream(dst)) {
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byte[] buf = new byte[BUFFER_SIZE];
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int n;
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while ((n = in.read(buf)) >= 0)
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out.write(buf, 0, n);
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}
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}
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// try-with-resources with a catch clause
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static String firstLineOfFile(String path, String defaultVal) {
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try (BufferedReader br = new BufferedReader(new FileReader(path))) {
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return br.readLine();
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} catch (IOException e) {
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return defaultVal;
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}
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}
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}
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