Java 7 新特性之try-with-resources实践理解

想象这么一个情景，我们需要使用一个资源，在使用完之后需要关闭该资源，并且使用该资源的过程中有可能有异常抛出。此时我们都会想到用try-catch语句，在finally中关闭该资源。此时会有一个问题，如果关闭该资源的时候也抛出了异常呢？见如下例子：

package com.rundong;

import java.io.IOException;

public class ResourceTest implements AutoCloseable{
    @Override
    public void close() throws IOException {
        throw new IOException("close Exception");
    }
}

package com.rundong;

import java.io.IOException;
import java.sql.SQLException;

public class Main {
    public static void main(String[] args) {
        try {
            throwTest();
        } catch (SQLException e) {
            System.out.println("catch SQLException");
            e.printStackTrace();
        } catch (IOException e) {
            System.out.println("catch IOException");
            e.printStackTrace();
        }
    }

    static void throwTest() throws SQLException, IOException {
        ResourceTest resource = null;
        try {
            resource = new ResourceTest();
            throw new SQLException("the first Exception");
        } catch (SQLException e) {
            System.out.println("catch the first Exception");
            // do something
            throw e;
        } finally {
            if (resource != null) resource.close();
        }
    }
}

输出结果：

发现什么问题了吗？

finally中抛出的异常将try中抛出的异常覆盖了！我们无法捕获到第一个异常！

Java 7 的解决方式：

package com.rundong;

import java.io.IOException;
import java.sql.SQLException;

public class Main {
    public static void main(String[] args) {
        try {
            throwTest();
        } catch (SQLException e) {
            System.out.println("catch SQLException");
            e.printStackTrace();
        } catch (IOException e) {
            System.out.println("catch IOException");
            e.printStackTrace();
        }
    }

    static void throwTest() throws SQLException, IOException {
        try (ResourceTest resource = new ResourceTest()) {
            throw new SQLException("the first Exception");
        } catch (SQLException e) {
            System.out.println("catch the first Exception");
            // do something
            throw e;
        }
    }
}