每天一分钟来一颗Java语法糖(五)try-with-resource

每天一分钟来一颗Java语法糖(五)try-with-resource

try-with-resource引入版本

• Java1.7增加了新特性：try-with-resource，资源必须定义在try中，若已经在外面定义，则需要一个本地变量。
• Java1.9 不再要求定义临时变量，可以直接使用外部资源变量。

try-with-resource介绍

程序如果打开外部资源，在使用后需要正确的关闭。Java提供了try-catch-finally机制，未异常等问题提供了保证。Java1.7以后提供了try-with-resource，它比try-catch-finall简便。

try-with-resource注意事项

• 资源对象必须实现AutoCloseable接口，即实现close方法
  如FileInputStream
  

try-with-resource演示

/**
  * 使用try-catch-finally方式的复制文件
  * @param source
  * @param target
  */
public static void copy(String source, String target) {
    InputStream in = null;
    OutputStream out = null;
    try {
        in = new FileInputStream(source);
        out = new FileOutputStream(target);
        byte[] buff = new byte[1024];
        int n;
        while ((n = in.read(buff)) >= 0) {
            out.write(buff, 0, n);
        }
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        if (in != null) {
            try {
                in.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        if (out != null) {
            try {
                out.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}
/**
 * 使用try-with-resource方式的复制文件
 * @param source
 * @param target
 */
public static void copy(String source, String target) {
    try (InputStream in = new FileInputStream(source);
         OutputStream out = new FileOutputStream(target)) {
        byte[] buff = new byte[1024];
        int n;
        while ((n = in.read(buff)) >= 0) {
            out.write(buff, 0, n);
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}
/**
 * 使用try-with-resource方式的复制文件，当然也可以不处理异常，异常让调用者自行处理
 * @param source
 * @param target
 */
public static void copy(String source, String target) throws IOException {
    try (InputStream in = new FileInputStream(source);
         OutputStream out = new FileOutputStream(target)) {
        byte[] buff = new byte[1024];
        int n;
        while ((n = in.read(buff)) >= 0) {
            out.write(buff, 0, n);
        }
    }
}

使用try-with-resource后，代码简洁，可读性增强，同时也不用在想着怎么手动关闭资源文件，有时候这些资源文件不关闭，搞不好就是一个线上事故。

try-with-resource其实是语法糖

try-with-resource其实是语法糖，它将在编译时编译成关闭资源的代码。
把刚才try-with-resource编译后的class文件反编译后如下

public static void copy(String source, String target) {
        try {
            InputStream in = new FileInputStream(source);
            Throwable var3 = null;

            try {
                OutputStream out = new FileOutputStream(target);
                Throwable var5 = null;

                try {
                    byte[] buff = new byte[1024];

                    int n;
                    while((n = in.read(buff)) >= 0) {
                        out.write(buff, 0, n);
                    }
                } catch (Throwable var31) {
                    var5 = var31;
                    throw var31;
                } finally {
                    if (out != null) {
                        if (var5 != null) {
                            try {
                                out.close();
                            } catch (Throwable var30) {
                                var5.addSuppressed(var30);
                            }
                        } else {
                            out.close();
                        }
                    }

                }
            } catch (Throwable var33) {
                var3 = var33;
                throw var33;
            } finally {
                if (in != null) {
                    if (var3 != null) {
                        try {
                            in.close();
                        } catch (Throwable var29) {
                            var3.addSuppressed(var29);
                        }
                    } else {
                        in.close();
                    }
                }

            }
        } catch (IOException var35) {
            var35.printStackTrace();
        }

    }