你还在使用 try-catch-finally 关闭资源？
你还在使用 try-catch-finally 关闭资源？
作者：何甜甜在吗 链接：https://juejin.im/post/5b8f9fa05188255c6f1df755
代码一定得写的优雅一点！
你还在使用try-catch-finally关闭资源吗，如果是，那么就有点out了。皮皮甜手把手教你使用JDK7引用的try-with-resource
JDK7之前资源的关闭姿势：
/**
* jdk7以前关闭流的方式
*
* @author hetiantian
* */
public class CloseResourceBefore7 {
private static final String FileName = "file.txt";
public static void main(String[] args) throws IOException {
FileInputStream inputStream = null;
try {
inputStream = new FileInputStream(FileName);
char c1 = (char) inputStream.read();
System.out.println("c1=" + c1);
} catch (IOException e) {
e.printStackTrace();
} finally {
if (inputStream != null) {
inputStream.close();
}
}
}
}
JDK7及以后关闭资源的正确姿势
try-with-resource Resource的定义：
所有实现了 java.lang.AutoCloseable[1] 接口（其中，它包括实现了 java.io.Closeable[2] 的所有对象），可以使用作为资源。简单Demo进行证实：实现java.lang.AutoCloseable接口的Resource类：
/**
* 资源类
*
* @author hetiantian
* */
public class Resource implements AutoCloseable {
public void sayHello() {
System.out.println("hello");
}
@Override
public void close() throws Exception {
System.out.println("Resource is closed");
}
}
测试类CloseResourceIn7.java
/**
* jdk7及以后关闭流的方式
*
* @author hetiantian
* */
public class CloseResourceIn7 {
public static void main(String[] args) {
try(Resource resource = new Resource()) {
resource.sayHello();
} catch (Exception e) {
e.printStackTrace();
}
}
}
打印结果：
hello
Resource is closed
当存在多个打开资源的时候：资源二Resource2.java
/**
* 资源2
*
* @author hetiantian
* */
public class Resource2 implements AutoCloseable {
public void sayhello() {
System.out.println("Resource say hello");
}
@Override
public void close() throws Exception {
System.out.println("Resource2 is closed");
}
}
测试类CloseResourceIn7.java
/**
* jdk7及以后关闭流的方式
*
* @author hetiantian
* */
public class CloseResourceIn7 {
public static void main(String[] args) {
try(Resource resource = new Resource(); Resource2 resource2 = new Resource2()) {
resource.sayHello();
resource2.sayhello();
} catch (Exception e) {
e.printStackTrace();
}
}
}
打印结果：
hello
Resource say hello
Resource2 is closed
Resource is closed
即使资源很多，代码也可以写的很简洁，如果用JDK7之前的方式去关闭资源，那么资源越多，用fianl关闭资源时嵌套也就越多。最近写的这篇：写了个全局变量的bug，也推荐看下。
那么它的底层原理又是怎样的呢，由皮皮甜独家揭秘优雅关闭资源背后的密码秘密
查看编译的class文件CloseResourceIn7.class：
public class CloseResourceIn7 {
public CloseResourceIn7() {
}
public static void main(String[] args) {
try {
Resource resource = new Resource();
Throwable var2 = null;
try {
resource.sayHello();
} catch (Throwable var12) {
var2 = var12;
throw var12;
} finally {
if (resource != null) {
if (var2 != null) {
try {
resource.close();
} catch (Throwable var11) {
var2.addSuppressed(var11);
}
} else {
resource.close();
}
}
}
} catch (Exception var14) {
var14.printStackTrace();
}
}
}
可以发现编译以后生成了try-catch-finally语句块 finally中的var2.addSuppressed(var11);
是不是有疑问？其实这么做是为了处理异常屏蔽的，我们将代码修改一下。
资源Resource.java
/**
* 资源类
*
* @author hetiantian
* */
public class Resource implements AutoCloseable {
public void sayHello() throws Exception {
throw new Exception("Resource throw Exception");
}
@Override
public void close() throws Exception {
throw new Exception("Close method throw Exception");
}
}
两个方法里面都抛出异常
测试类CloseResourceIn7.java
/**
* jdk7及以后关闭流的方式
*
* @author hetiantian
* */
public class CloseResourceIn7 {
public static void main(String[] args) {
try {
errorTest();
} catch (Exception e) {
e.printStackTrace();
}
}
private static void errorTest() throws Exception {
Resource resource = null;
try {
resource = new Resource();
resource.sayHello();
}
finally {
if (resource != null) {
resource.close();
}
}
}
}
打印结果：
java.lang.Exception: Close method throw Exception
at com.shuwen.Resource.close(Resource.java:15)
at com.shuwen.CloseResourceIn7.errorTest(CloseResourceIn7.java:27)
at com.shuwen.CloseResourceIn7.main(CloseResourceIn7.java:12)
只打印了最后出现的异常【异常屏蔽】这样会给开发人员排查错误带来一定的困难 我们换成try-with-resource方法实现CloseResourceIn7.java
/**
* jdk7及以后关闭流的方式
*
* @author hetiantian
* */
public class CloseResourceIn7 {
public static void main(String[] args) {
try {
errorTest();
} catch (Exception e) {
e.printStackTrace();
}
}
private static void errorTest() throws Exception {
try(Resource resource = new Resource()) {
resource.sayHello();
}
}
}
打印信息：
java.lang.Exception: Resource throw Exception
at com.shuwen.Resource.sayHello(Resource.java:10)
at com.shuwen.CloseResourceIn7.errorTest(CloseResourceIn7.java:20)
at com.shuwen.CloseResourceIn7.main(CloseResourceIn7.java:12)
Suppressed: java.lang.Exception: Close method throw Exception
at com.shuwen.Resource.close(Resource.java:15)
at com.shuwen.CloseResourceIn7.errorTest(CloseResourceIn7.java:21)
... 1 more
可以发现，异常信息中多了一个Suppressed的提示，告诉我们这个异常其实由两个异常组成，Close method throw Exception这个异常是被Suppressed【屏蔽】的异常
参考资料
[1]
java.lang.AutoCloseable: http://docs.oracle.com/javase/8/docs/api/java/lang/AutoCloseable.html
[2]
java.io.Closeable: http://docs.oracle.com/javase/8/docs/api/java/io/Closeable.html