假设有两个类实现AutoCloseable
接口,如下所示:
public class Closing1 implements AutoCloseable {
private boolean closed;
@Override
public void close() throws Exception {
if (closed) {
throw new Exception("Closed Already");
}
this.closed = true;
System.out.println("Closing1 closed");
}
public boolean isClosed() {
return closed;
}
}
和
public class Closing2 implements AutoCloseable {
private Closing1 cl1;
public Closing2(Closing1 cl1) {
this.cl1 = cl1;
}
@Override
public void close() throws Exception {
if(!cl1.isClosed()) {
throw new Exception("Closing1 not closed");
}
System.out.println("Closing2 closed");
}
}
我发现尝试使用资源的所有变化都会导致异常!我在这里缺少什么,还是只是 TWR 的设计方式?
try(Closing1 c1 = new Closing1();Closing2 c2 = new Closing2(c1)){
System.out.println("Done");
} //Exception while auto closing C2
或
try(Closing1 c1 = new Closing1();Closing2 c2 = new Closing2(c1)){
System.out.println("Done");
c1.close();
} // exception while auto closing c1
Try-with-resources 将按照与其声明相反的顺序关闭资源。这意味着将首先调用c2.close()
,这将在您编码时引发异常。
首先从尝试资源开始,https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html
如第一个示例所示:
try (BufferedReader br = new BufferedReader(new FileReader(path))) {
return br.readLine();
}
人们不一定会说出链条中的所有东西。
除非你明确需要 c1 做某事(除了关闭(,否则在现实生活中,你的片段宁愿看起来像
try(Closing2 c2 = new Closing2(new Closing1())){
System.out.println("Done");
}
而且您肯定不会在 try-block 中调用c1.close()
,因为根本没有 C1。
记住这一点,因为包含的 c1 没有关闭而从 c2 抛出异常是完全错误的,实际上 c2 拥有 Closesing1 对象,应该在其上调用close()
:
class Close1 implements AutoCloseable {
@Override
public void close() throws Exception {
System.out.println("Closing c1");
}
}
class Close2 implements AutoCloseable {
Close1 c1;
Close2(Close1 c1) {
this.c1=c1;
}
@Override
public void close() throws Exception {
System.out.print("Closing c1 from c2: ");
c1.close();
System.out.println("Closing c2");
}
}
void test() {
System.out.println("Before try block");
try(Close2 c2=new Close2(new Close1())) {
System.out.println("In try block");
}
catch(Exception ex) {
System.out.println("Exception: "+ex);
}
finally {
System.out.println("In finally block");
}
System.out.println("After try block");
}
但是,如果有人给 c1 起一个名字,它将被关闭两次,这就是幂等性出现的地方,正如有人已经建议的那样:
System.out.println("Before try block");
try(Close1 c1 = new Close1(); Close2 c2 = new Close2(c1)){
System.out.println("In try block");
}
catch(Exception ex){
System.out.println("Exception: "+ex);
}
finally{
System.out.println("In finally block");
}
System.out.println("After try block");
正如已经提到的BufferedReader
,这是它close()
方法:
public void close() throws IOException {
synchronized (lock) {
if (in == null)
return;
try {
in.close();
} finally {
in = null;
cb = null;
}
}
}
如果它有in
,它将被关闭并空(在 finally 块中,因此即使发生异常也会发生(,并且全部在线程安全块中。(cb
只是一个字符数组,它也为空,稍微简化了垃圾收集器的生活(。由于将 finally 块中的所有内容清空,因此对同一方法的任何额外调用都不会执行任何操作(除了在锁上同步片刻(。