諸如流哪亿、文件等系統(tǒng)資源檩电，當(dāng)其被打開時(shí)，需要手動(dòng)關(guān)閉现斋，要不必將引起一場大災(zāi)害喜最。

傳統(tǒng)的try cache finally

在java7之前，我們可以使用finally來確保資源被關(guān)閉庄蹋，例如：

static String readFirstLineFromFileWithFinallyBlock(String path) throws IOException {
  BufferedReader br = new BufferedReader(new FileReader(path));
  try {
    return br.readLine();
  } finally {
    if (br != null) 
        br.close();
  }
}

但是如果br.readLine()有異常瞬内，br.close()也有異常拋出迷雪，由于一次只能拋一次異常，try的異常會(huì)被抑制虫蝶，finally的異常會(huì)被拋出章咧，導(dǎo)致異常丟失。以下是具體演示：

public class Connection implements AutoCloseable {

    public void sendData() throws Exception {
        throw new Exception("send data");
    }
    @Override
    public void close() throws Exception {
        throw new Exception("close");
    }
}

public class TryWithResource {

    public static void main(String[] args) {
        try{
            test();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private static void test()throws Exception{
        Connection conn = null;
        try{
            conn = new Connection();
            conn.sendData();
        } finally{
            if(conn !=null) {
                conn.close();
            }
        }
    }
}

運(yùn)行結(jié)果如下：

java.lang.Exception: close
    at cn.wangjy.study.trywithresources.Connection.close(Connection.java:18)
    at cn.wangjy.study.trywithresources.TryWithResource.test(TryWithResource.java:84)
    at cn.wangjy.study.trywithresources.TryWithResource.main(TryWithResource.java:71)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

用try-with-resources實(shí)現(xiàn)

JAVA 7之后能真，我們盼來福音赁严，再也不需要在finally里面寫一堆if（xxx !=null）xxx.close()的代碼，只要資源實(shí)現(xiàn)了AutoCloseable或者Closeable接口舟陆，try-with-resources能幫其自動(dòng)關(guān)閉误澳。將以上的代碼改寫為try-with-resources

public class TryWithResource {

    public static void main(String[] args) {
        try(Connection conn =new Connection()) {
            conn.sendData();
        }catch(Exception e) {
            e.printStackTrace();
        }
    }
}

運(yùn)行結(jié)果如下：

java.lang.Exception: send data
    at cn.wangjy.study.trywithresources.Connection.sendData(Connection.java:13)
    at cn.wangjy.study.trywithresources.TryWithResource.main(TryWithResource.java:71)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
    Suppressed: java.lang.Exception: close
        at cn.wangjy.study.trywithresources.Connection.close(Connection.java:18)
        at cn.wangjy.study.trywithresources.TryWithResource.main(TryWithResource.java:72)
        ... 5 more

可以看到耻矮，代碼編寫也十分簡潔秦躯。雖然我們沒有用finally的代碼塊寫明close()，但是程序仍然會(huì)自動(dòng)執(zhí)行close()方法裆装，并且異常信息也沒有丟失踱承。信息中多了一個(gè)Suppressed的提示，附帶上close的異常哨免。這個(gè)異常其實(shí)由兩個(gè)異常組成茎活， 執(zhí)行close()的異常是被Suppressed的異常。

原理是什么呢琢唾？

反編譯了TryWithResource的class文件载荔，代碼如下

public static void main(String[] args) {
        try {
            Connection conn = new Connection();
            Throwable localThrowable3 = null;
            try {
                conn.sendData();
            } catch (Throwable localThrowable1) {
                localThrowable3 = localThrowable1;
                throw localThrowable1;
            } finally {
                if (conn != null) if (localThrowable3 != null) try {
                    conn.close();
                } catch (Throwable localThrowable2) {
                    localThrowable3.addSuppressed(localThrowable2);
                }
                else conn.close();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

可見，編譯器在編譯的時(shí)候自動(dòng)幫我們加了finally塊采桃，并添加了資源的close方法懒熙，所以例子的close會(huì)在運(yùn)行的時(shí)候被執(zhí)行。細(xì)心的你發(fā)現(xiàn)了嗎普办，代碼中還多了localThrowable3.addSuppressed(localThrowable2)的方法工扎。java7之后，Throwable類新增了addSuppressed方法衔蹲，支持將一個(gè)異常附加到另一個(gè)異常身上肢娘，從而避免異常屏蔽。這樣輸出的異常信息會(huì)附帶Suppressed的提示舆驶，參考上圖的運(yùn)行結(jié)果橱健。

try-with-resources 也支持聲明多個(gè)資源

public static void main(String[] args) {
        try (FileInputStream fin = new FileInputStream(new File("input.txt")); 
             FileOutputStream fout = new FileOutputStream(new File("out.txt"));
             GZIPOutputStream out = new GZIPOutputStream(fout)) {
            byte[] buffer = new byte[4096];
            int read;
            while ((read = fin.read(buffer)) != -1) {
                out.write(buffer, 0, read);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

資源關(guān)閉會(huì)按聲明時(shí)的相反順序被執(zhí)行。

參考：
http://docs.oracle.com/javase/7/docs/technotes/guides/language/try-with-resources.html
http://www.tuicool.com/articles/JvQRBri