java中try-with-resources自动关闭io流

java中try-with-resources自动关闭io流

0 简要说明

在传统的输入输出流处理中，我们一般使用的结构如下所示，使用try - catch - finally结构捕获相关异常，最后不管是否有异常，我们都将流进行关闭处理

  try {
           //业务代码
        } catch (SQLException e) {
            // 异常，数据回滚
            try {
                connection.rollback();
            } catch (SQLException ex) {
                log.error(ERROR_MSG, e);
                throw new IllegalArgumentException(e);
            }
            log.error(ERROR_MSG, e);
            throw new IllegalArgumentException(e);
        } finally {
            if (Objects.nonNull(preparedStatement)) {
                try {
                    preparedStatement.close();
                } catch (SQLException e) {
                    log.error(ERROR_MSG, e);
                    throw new IllegalArgumentException(e);
                }
            }
            if (Objects.nonNull(connection)) {
                try {
                    connection.close();
                } catch (SQLException e) {
                    log.error(ERROR_MSG, e);
                    throw new IllegalArgumentException(e);
                }
            }
        }

try-with-resources

在jdk1.7之后，推荐使用try() {} catch(IOException e){}的方式来处理io流，它可以自动关闭流。如下所示，是一个简单的按行读取文件内容的示例

 @Test
    public void readFileContent() {
        File file = new File("d://DumpStacks.log");
        try (BufferedReader reader = new BufferedReader(new FileReader(file))) {
            String line;
            while ((line = reader.readLine()) != null) {
                log.info("line : 【{}】" , line);
            }
        } catch (IOException e) {
            log.error("error msg : 【{}】", e);
            throw new RuntimeException(e);
        }
    }

抛异常情况

java.lang.RuntimeException: java.io.FileNotFoundException: d:\DumpStacks.log (系统找不到指定的文件。)

at com.geekmice.springbootselfexercise.NoDaoTest.readFileContent(NoDaoTest.java:347)
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:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)

Caused by: java.io.FileNotFoundException: d:\DumpStacks.log (系统找不到指定的文件。)
at java.io.FileInputStream.open0(Native Method)
at java.io.FileInputStream.open(FileInputStream.java:195)
at java.io.FileInputStream.(FileInputStream.java:138)
at java.io.FileReader.(FileReader.java:72)
at com.geekmice.springbootselfexercise.NoDaoTest.readFileContent(NoDaoTest.java:340)

反编译之后可以看到自动关闭流操作

打开target，找到对应字节码文件

 @Test
    public void readFileContent() {
        File file = new File("d://DumpStascks.log");

        try {
            BufferedReader reader = new BufferedReader(new FileReader(file));
            Throwable var3 = null;

            try {
                String line;
                try {
                    while((line = reader.readLine()) != null) {
                        log.info("line : 【{}】", line);
                    }
                } catch (Throwable var13) {
                    var3 = var13;
                    throw var13;
                }
            } finally {
                if (reader != null) {
                    if (var3 != null) {
                        try {
                            reader.close();
                        } catch (Throwable var12) {
                            var3.addSuppressed(var12);
                        }
                    } else {
                        reader.close();
                    }
                }

            }

        } catch (IOException var15) {
            log.error("error msg : 【{}】", var15);
            throw new RuntimeException(var15);
        }
    }

当使用了try(){}代码块之后，代码执行完毕，就可以进行流的自动关闭，比传统的方式简洁了不少。
try()这部分，可以有多个语句，语句之间分号隔开，也可以同时包含输入流和输出流。最后执行完毕，统一关闭。