java：AutoCloseable接口使用
1 前言
AutoCloseable接口，在try-with-resources代码块退出时，会自动调用接口的close方法，避免资源耗尽的异常等情况。
参考java文档，如下：
An object that may hold resources (such as file or socket handles) until it is closed.
The close() method of an AutoCloseable object is called automatically
when exiting a 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.
2 使用
public class TestAutoC {
public void run1(){
try(ACL acl = new ACL(); BCL bcl = new BCL()){
acl.run();
bcl.run();
} catch (Exception e) {
e.printStackTrace();
}
}
public void run2(){
ACL acl = new ACL();
BCL bcl = new BCL();
try {
acl.run();
bcl.run();
} catch (Exception e) {
e.printStackTrace();
}
}
public void run3(){
ACL acl;
BCL bcl;
try {
acl = new ACL();
bcl = new BCL();
acl.run();
bcl.run();
} catch (Exception e) {
e.printStackTrace();
}
}
public void run4(){
try(ACL acl = new ACL(); BCL bcl = new BCL()){
acl.run();
int i = 0;
if(i == 0){
throw new RuntimeException("unknown error");
}
bcl.run();
}
catch (Exception e) {
e.printStackTrace();
throw (RuntimeException)e;
}
}
public static void main(String[] args) {
TestAutoC autoC = new TestAutoC();
autoC.run1();
System.out.println("*************");
autoC.run2();
System.out.println("*************");
autoC.run3();
System.out.println("*************");
autoC.run4();
}
}
class ACL implements AutoCloseable{
{
System.out.println("ACL instance init");
}
public void run(){
System.out.println("ACL run");
}
@Override
public void close() throws Exception {
System.out.println("ACL close");
}
}
class BCL implements AutoCloseable{
{
System.out.println("BCL instance init");
}
public void run(){
System.out.println("BCL run");
}
@Override
public void close() throws Exception {
System.out.println("BCL close");
}
}
执行结果可知，在try-with-resources代码块结束时才会自动调用close方法（try代码块中的方法已执行），而仅仅在try代码块中执行（非try-with-resources）不会自动调用close方法，且使用了try-with-resources代码块时，即便catch抛出异常，也会执行close方法：
ACL instance init
BCL instance init
ACL run
BCL run
BCL close
ACL close
*************
ACL instance init
BCL instance init
ACL run
BCL run
*************
ACL instance init
BCL instance init
ACL run
BCL run
*************
ACL instance init
BCL instance init
ACL run
BCL close
ACL close
java.lang.RuntimeException: unknown error
at com.xiaoxu.test.TestAutoC.run4(TestAutoC.java:48)
at com.xiaoxu.test.TestAutoC.main(TestAutoC.java:67)
Exception in thread "main" java.lang.RuntimeException: unknown error
at com.xiaoxu.test.TestAutoC.run4(TestAutoC.java:48)
at com.xiaoxu.test.TestAutoC.main(TestAutoC.java:67)
并且上述close方法调用的顺序是，try-with-resources代码块中顺序定义靠后的资源，优先调用close方法释放。在使用try-with-resources代码块时，须加上catch语句，用于捕获close方法的Exception异常，否则提示：Unhandled exception from auto-closeable resource: java.lang.Exception（或者方法直接显示声明throws Exception，在外层逻辑接收此异常）。
当然，如果资源使用时，需要添加其他操作，那么可以采用在try代码块中嵌入另外一个资源的try代码块的方式，依然可以达到最外层嵌套的try-with-resources代码块最后调用close方法的效果：
public void run5() throws Exception{
try(ACL acl = new ACL()){
acl.run();
System.out.println("acl run over");
try(BCL bcl = new BCL()){
bcl.run();
}
System.out.println("bcl run over");
}
}
执行结果如下：
ACL instance init
ACL run
acl run over
BCL instance init
BCL run
BCL close
bcl run over
ACL close
需注意，若使用嵌套的try-with-resources代码块释放资源时，若外层调用抛出异常，则内部的try-with-resources代码块还未开始执行，也就不会有资源的定义和释放，若内层try-with-resources代码块抛出异常，也不会影响内层和外层已定义资源的释放，所以不会出现问题，如下可示：
示例1：
public void run6() throws Exception{
try(ACL acl = new ACL()){
acl.run();
int i = 0;
if(i == 0){
throw new RuntimeException("unknown error");
}
try(BCL bcl = new BCL()){
bcl.run();
}
}
}
执行结果：
ACL instance init
ACL run
ACL close
Exception in thread "main" java.lang.RuntimeException: unknown error
at com.xiaoxu.test.TestAutoC.run6(TestAutoC.java:81)
at com.xiaoxu.test.TestAutoC.main(TestAutoC.java:114)
示例2：
public void run7() throws Exception{
try(ACL acl = new ACL()){
acl.run();
int i = 0;
try(BCL bcl = new BCL()){
bcl.run();
if(i == 0){
throw new RuntimeException("unknown error");
}
}
}
}
执行结果：
ACL instance init
ACL run
BCL instance init
BCL run
BCL close
ACL close
Exception in thread "main" java.lang.RuntimeException: unknown error
at com.xiaoxu.test.TestAutoC.run7(TestAutoC.java:70)
at com.xiaoxu.test.TestAutoC.main(TestAutoC.java:117)
实际运用场景中，比如RabbitMQ消息中，根据官方文档亦有提示，
The connection abstracts the socket connection, and takes care of protocol version negotiation and authentication and so on for us. Here we connect to a RabbitMQ node on the local machine - hence the localhost. If we wanted to connect to a node on a different machine we'd simply specify its hostname or IP address here.
Next we create a channel, which is where most of the API for getting things done resides. Note we can use a try-with-resources statement because both Connection and Channel implement java.lang.AutoCloseable. This way we don't need to close them explicitly in our code.
To send, we must declare a queue for us to send to; then we can publish a message to the queue, all of this in the try-with-resources statement:
即RabbitMQ中，获取Connection和Channel都可以在try-with-resources代码块中使用，因为它们都实现了java.lang.AutoCloseable接口，无需再显示的调用close方法了，其余常见的使用场景，还有数据库的Connection、Statement、ResultSet等资源的自动关闭等等：
ConnectionFactory factory = new ConnectionFactory();
factory.setHost("localhost");
try (Connection connection = factory.newConnection();
Channel channel = connection.createChannel()) {
}
RabbitMQ官网地址：
https://www.rabbitmq.com/
https://www.rabbitmq.com/tutorials/tutorial-one-java.html
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