面试题
代码语言：javascript复制import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
/**
* FileInputStream在使用完以后，不关闭流，想二次使用可以怎么操作？
* @author dhb
*/
public class FileInputStreamDemo {
public static void main(String[] args) throws IOException, NoSuchMethodException, InvocationTargetException, IllegalAccessException, NoSuchFieldException {
String parentPath = "";
String text1Path = parentPath "text1.txt";
String text2Path = parentPath "text2.txt";
String text3Path = parentPath "text3.txt";
FileInputStream text1InputStream = new FileInputStream(text1Path);
FileOutputStream text2OutputStream = new FileOutputStream(text2Path);
FileOutputStream text3OutputStream = new FileOutputStream(text3Path);
int len;
byte[] bytes = new byte[1024];
while ((len = text1InputStream.read(bytes)) != -1) {
text2OutputStream.write(bytes, 0, len);
}
Class<? extends FileInputStream> inputStreamClass = text1InputStream.getClass();
Field fd = inputStreamClass.getDeclaredField("fd");
fd.setAccessible(true);
Object o = fd.get(text1InputStream);
System.out.println(o.hashCode());
if (text1InputStream.read() == -1) {
Method open0 = inputStreamClass.getDeclaredMethod("open0", String.class);
open0.setAccessible(true);
open0.invoke(text1InputStream, text1Path);
}
Object o1 = fd.get(text1InputStream);
System.out.println(o1.hashCode());
while ((len = text1InputStream.read(bytes)) != -1) {
text3OutputStream.write(bytes, 0, len);
}
text2OutputStream.flush();
text2OutputStream.close();
text3OutputStream.flush();
text3OutputStream.close();
}
}open和read执行流程
总结
FileInputStream通过再次open能实现重读的原理是：底层代码重新打开了一次文件，返回文件描述符(Linux)或句柄，这个值存在FileInputStream中的一个类型为FileDescriptor的对象中，FileDescriptor的fd是文件描述符，handle是句柄
