背景

在日常開發(fā)中，經(jīng)常都是手動(dòng)關(guān)閉資源刊橘，比如IO流鄙才，數(shù)據(jù)庫(kù)連接等。如果忘關(guān)了就可能造成嚴(yán)重的性能后果促绵。

try-finally帶來的問題

1攒庵、代碼不優(yōu)雅

手動(dòng)關(guān)閉IO流，特別是使用多個(gè)時(shí)败晴，finally方法里寫的嵌套代碼真的是不堪入目浓冒，下面看一個(gè)例子：

 public static void main(String[] args) {
 
        BufferedInputStream inputStream = null;
        BufferedOutputStream outputStream = null;
        try {
            inputStream = new BufferedInputStream(new FileInputStream("t1.txt"));
            outputStream = new BufferedOutputStream(new FileOutputStream("t2.txt"));
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                inputStream.close();
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                try {
                    outputStream.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
 
            }
        }
    }

因?yàn)橐ＷCinputStream和outputStream都要正常關(guān)閉，所以往往會(huì)選擇嵌套著寫尖坤。

如果finally寫成下面那樣：

public static void main(String[] args) {
 
        BufferedInputStream inputStream = null;
        BufferedOutputStream outputStream = null;
        try {
            inputStream = new BufferedInputStream(new FileInputStream("t1.txt"));
            outputStream = new BufferedOutputStream(new FileOutputStream("t2.txt"));
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                inputStream.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            try {
                outputStream.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

當(dāng)inputStream.close(); 出現(xiàn)異常了稳懒，就不會(huì)再運(yùn)行 outputStream.close();了，就會(huì)一直無法釋放資源慢味。

2场梆、異常屏蔽問題

當(dāng)我們想向上層throws異常時(shí)，由于只能拋出一個(gè)異常纯路，所以下面例子中或油，inputStream的異常就被outStream屏蔽了。

控制臺(tái)只顯示outStream的異常：

894512e0b76343dfad5496735360c9e0_tplv-k3u1fbpfcp-watermark.png

活用try-with-resource

Java 7后引人 try-with-resources驰唬，幫助我們快速解決資源釋放顶岸。

上述代碼使用try-with-resource，是不是優(yōu)雅了很多＝斜唷辖佣！

public static void main(String[] args) {
        try (
                BufferedInputStream inputStream = new BufferedInputStream(new FileInputStream("t1.txt"));
                BufferedOutputStream outputStream = new BufferedOutputStream(new FileOutputStream("t2.txt"));
        ) {
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

其實(shí)try-with-resource的原理就是幫我們自動(dòng)補(bǔ)全close，下面是編譯后的class搓逾，和我們try-finally寫的是大同小異凌简。

public static void main(String[] args) {
        try {
            BufferedInputStream inputStream = new BufferedInputStream(new FileInputStream("t1.txt"));
 
            try {
                BufferedOutputStream outputStream = new BufferedOutputStream(new FileOutputStream("t2.txt"));
                outputStream.close();
            } catch (Throwable var5) {
                try {
                    inputStream.close();
                } catch (Throwable var4) {
                    var5.addSuppressed(var4);
                }
 
                throw var5;
            }
 
            inputStream.close();
        } catch (Exception var6) {
            var6.printStackTrace();
        }
 
    }