最近在做代码审核的时候，审核工具提示我将 try-catch-finally 给替换掉，而且根据公司相关要求，该提示的级别还不低，不改不予通过。
FileReader fr = null;
BufferedReader br = null;
try {
fr = new FileReader(fileName);
br = new BufferedReader(fr);
return br.readLine();
} catch (Exception e) {
log.error("error:{}", e);
} finally {
if (br != null) {
try {
br.close();
} catch(IOException e){
log.error("error:{}", e);
}
}
if (fr != null ) {
try {
br.close();
} catch(IOException e){
log.error("error:{}", e);
}
}
}
try (
FileReader fr = new FileReader(fileName);
BufferedReader br = new BufferedReader(fr)
) {
return br.readLine();
}catch (Exception e) {
log.error("error:{}", e);
}
try (
BufferedReader br = new BufferedReader(new FileReader(fileName))
) {
// no need to name intermediate resources if you don't want to
return br.readLine();
}
catch (Exception e) {
log.error("error:{}", e);
}
对比代码，不难发现，输入输出流的关闭存在着差异。难道输入输出流不用关闭了吗？
public class FileInputStream extends InputStream{}
public abstract class InputStream implements Closeable {}
/**
* A {@code Closeable} is a source or destination of data that can be closed.
* The close method is invoked to release resources that the object is
* holding (such as open files).
*
* @since 1.5
*/
public interface Closeable extends AutoCloseable {}
/**
* An object that may hold resources (such as file or socket handles)
* until it is closed. The {@link #close()} method of an {@code AutoCloseable}
* object is called automatically when exiting a {@code
* try}-with-resources block for which the object has been declared in
* the resource specification header. This construction ensures prompt
* release, avoiding resource exhaustion exceptions and errors that
* may otherwise occur.
*
* @apiNote
* <p>It is possible, and in fact common, for a base class to
* implement AutoCloseable even though not all of its subclasses or
* instances will hold releasable resources. For code that must operate
* in complete generality, or when it is known that the {@code AutoCloseable}
* instance requires resource release, it is recommended to use {@code
* try}-with-resources constructions. However, when using facilities such as
* {@link java.util.stream.Stream} that support both I/O-based and
* non-I/O-based forms, {@code try}-with-resources blocks are in
* general unnecessary when using non-I/O-based forms.
*
* @author Josh Bloch
* @since 1.7
*/
public interface AutoCloseable {}
AutoCloseable 顾名思义， 自动关闭流. 从注释中我们可以发现，实现了AutoCloseable并在try()中声明的对象，当try-with-resource代码块执行完的时候，会自动调用close()方法。
注意：
一个 try-with-resources 语句可以像普通的 try 语句那样有 catch 和 finally 块。在try-with-resources 语句中， 任意的 catch 或者 finally 块都是在声明的资源被关闭以后才运行。
try-with-resource是JDK7引入的语法糖，可以简化Autocloseable资源类的关闭过程，
File file = new File("d:/tmp/1.txt");
FileInputStream fis = null;
try {
fis = new FileInputStream(file);
xxxxx
xxxxx
} catch (IOException e) {
e.printStackTrace();
}finally{
if(fis != null){
try {
fis.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
上面是一段读取文件内容的示意代码，为了防止在try代码块中出现异常后导致的资源泄露问题，在finally代码块中一般处理资源的关闭事项。
File file = new File("d:/tmp/1.txt");
try(FileInputStream fis = new FileInputStream(file);) {
fis.read();
} catch (IOException e) {
e.printStackTrace();
}finally{
}
可以看出是简化了不少，之所以称之为语法糖，是因为编译成class文件后实际的代码就不是这样的了，编译过程中会自动添加资源的关闭处理。
File file = new File("d:/tmp/1.txt");
try {
Throwable var2 = null;
Object var3 = null;
try {
FileInputStream fis = new FileInputStream(file);
xxx
xxxx
} catch (Throwable var12) {
if (var2 == null) {
var2 = var12;
} else if (var2 != var12) {
var2.addSuppressed(var12);
}
throw var2;
}
} catch (IOException var13) {
var13.printStackTrace();
}
好了，上面已经引入今天的主题，try-with-resource，但是仍然有需要注意的地方。
private static class MyResource implements AutoCloseable{
private MyResource1 res;
public MyResource(MyResource1 res){
this.res = res;
}
@Override
public void close() throws Exception {
System.out.println("MyResource自动关闭");
Integer a = null;
a.toString();
this.res.close();
}
}
private static class MyResource1 implements AutoCloseable{
@Override
public void close() throws Exception {
System.out.println("MyResource1自动关闭");
}
}
@Test
public void test() throws Exception{
try(
MyResource r = new MyResource(new MyResource1())){
Integer a = null ;
a.toString();
}
}
执行上面的代码，由于MyResource的close方法中出现了异常，此时创建的MyResource1就不会被关闭，从而出现资源泄露情况，为了规避这个问题，为了规避这个问题，我们需要创建的实现AutoCloseable接口的对象单独创建。
try(
MyResource1 res= new MyResource1();
MyResource r = new MyResource(res)){
Integer a = null ;
a.toString();
}
以上为个人经验，希望能给大家一个参考，也希望大家多多支持。
