JDK7新特性try-with-resources语句

try-with-resources语句是一种声明了一种或多种资源的try语句。资源是指在程序用完了之后必须要关闭的对象。try-with-resources语句保证了每个声明了的资源在语句结束的时候都会被关闭。任何实现了java.lang.AutoCloseable接口的对象，和实现了java.io.Closeable接口的对象，都可以当做资源使用。

下面是使用样例

1.申明两个实现AutoCloseable接口的类

package jdk7_new_character;

public class AutoCloseableTestClass implements AutoCloseable {

    @Override
    public void close() throws Exception {
        System.out.println("autoclose AutoCloseableTestClass");
    }

}

package jdk7_new_character;

public class AutoCloseableTestClass2 implements AutoCloseable {

    @Override
    public void close() throws Exception {
        System.out.println("autoclose AutoCloseableTestClass2");

    }

}

2.测试

package jdk7_new_character;

public class Test {

    @org.junit.Test
    public void test() {
        // fail("Not yet implemented");
    }

    @org.junit.Test
    public void testAutoClose() {
        System.out.println("begin testAutoClose");
        /*
         * try { AutoCloseableTestClass autoCloseableTestClass = new
         * AutoCloseableTestClass(); } catch (Exception e) { } finally {
         *
         * }
         */

        try (AutoCloseableTestClass autoCloseableTestClass = new AutoCloseableTestClass();
             AutoCloseableTestClass2 autoCloseableTestClass2 = new AutoCloseableTestClass2()
        ) {
            System.out.println("try{}");
        } catch (Exception e) {
            e.printStackTrace();
        }

        System.out.println("over testAutoClose");
    }

}

3.运行结果

可以看到你可以在一个try-with-resources语句里面声明多个资源。当代码块中代码终止，不管是正常还是异常，对象的close方法都会自动按声明的相反顺序调用。

注意：try-with-resources语句也可以像普通的try语句一样，有catch和finally代码块。在try-with-resources语句中，任何的catch和finally代码块都在所有被声明的资源被关闭后执行。

被压抑的异常

try-with-resources语句相关联的代码块可能会抛出异常。try代码块中可能会抛出异常，并且有高达两个异常可能会在try-with-resources语句抛出。如果在try代码块中抛出一个异常，同时在try-with-resources语句中抛出一个或多个异常，那么在try-with-resources语句中抛出的异常就被压抑了，并且最终在方法抛出的异常就是try代码块中抛出的那个异常。你可以通过被try代码块抛出的异常的Throwable.getSuppressed方法找回被压抑的异常。

下面是测试

1.被压抑的异常

申明两个实现AutoCloseable接口的class，并在close方法中抛出异常

package jdk7_new_character;

/**
 * @author 施俊杰
 */
public class AutoCloseableThrowableTestClass implements AutoCloseable {

    @Override
    public void close() throws Exception {
        System.out.println("close AutoCloseableThrowableTestClass");
        throw new Exception("AutoCloseableThrowableTestClass exception");
    }

}

package jdk7_new_character;

/**
 * @author 施俊杰
 */
public class AutoCloseableThrowableTestClass2 implements AutoCloseable {

    @Override
    public void close() throws Exception {
        System.out.println("close AutoCloseableThrowableTestClass2");
        throw new Exception("AutoCloseableThrowableTestClass2 exception");
    }

}

创建测试class去创建上面申明的两个类

package jdk7_new_character;

public class ThrowableTestClass {

    public String testThrowable() throws Exception {

        try (AutoCloseableThrowableTestClass autoCloseableThrowableTestClass = new AutoCloseableThrowableTestClass();
                AutoCloseableThrowableTestClass2 autoCloseableThrowableTestClass2 = new AutoCloseableThrowableTestClass2()) {
            throw new Exception("exception3");
        }

    }

    public static void main(String[] args) {
        try {
            new ThrowableTestClass().testThrowable();
        } catch (Exception e) {
            System.out.println(e.getMessage());

        }
    }

}

运行结果

可以看到

AutoCloseableThrowableTestClass2 exception
AutoCloseableThrowableTestClass exception

两个exception被压抑了，我们可以通过Throwable.getSuppressed方法找回被压抑的异常，示例代码如下

package jdk7_new_character;

public class ThrowableTestClass {

    public String testThrowable() throws Exception {

        try (AutoCloseableThrowableTestClass autoCloseableThrowableTestClass = new AutoCloseableThrowableTestClass();
                AutoCloseableThrowableTestClass2 autoCloseableThrowableTestClass2 = new AutoCloseableThrowableTestClass2()) {
            throw new Exception("exception3");
        }

    }

    public static void main(String[] args) {
        try {
            new ThrowableTestClass().testThrowable();
        } catch (Exception e) {
            System.out.println(e.getMessage());
            Throwable[] suppressed = e.getSuppressed();
            for (Throwable throwable : suppressed) {
                System.out.println(throwable.getMessage());
            }
        }
    }

}

运行结果

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