removeEldestEntry覆盖
如何覆盖removeEldestEntry方法以将最长的条目保存到文件？另外如何限制文件的大小，就像我在LinkedHashMap中所做的那样。这是代码：
import java.util.*;
public class level1 {
private static final int max_cache = 50;
private Map cache = new LinkedHashMap(max_cache, .75F, true) {
protected boolean removeEldestEntry(Map.Entry eldest) {
return size() > max_cache;
}
};
public level1() {
for (int i = 1; i < 52; i++) {
String string = String.valueOf(i);
cache.put(string, string);
System.out.println("rCache size = " + cache.size() +
"tRecent value = " + i + " tLast value = " +
cache.get(string) + "tValues in cache=" +
cache.values());
}
private Map cache = new LinkedHashMap(max_cache, .75F, true) {
protected boolean removeEldestEntry(Map.Entry eldest) throws IOException {
boolean removed = super.removeEldestEntry(eldest);
if (removed) {
FileOutputStream fos = new FileOutputStream("t.tmp");
ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.writeObject(eldest.getValue());
oos.close();
}
return removed;
}
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2 个回复
葛瞎说漓
。有几种方法可以做到这一点，将其转换为某种类型的和/或记录它。 如果你担心文件大小，你可能不想只保留最后删除的条目，但很多 - 所以你应该打开文件进行追加。 你需要从方法返回来实际删除eldest，你需要决定是否应该删除该元素。 使用文件时，您应该使用try / finally确保即使存在异常也关闭资源。这可能会有点难看 - 有时候有一个实用工具方法来完成关闭很好，所以你不需要额外的try / catch。 通常，您还应该对文件I / O使用一些缓冲，这可以大大提高性能;在这种情况下，使用将文件流包装在中并将其提供给。 这可能是你想做的事情：private static final int MAX_ENTRIES_ALLOWED = 100; private static final long MAX_FILE_SIZE = 1L * 1024 * 1024; // 1 MB protected boolean removeEldestEntry(Map.Entry eldest) { if (size() <= MAX_ENTRIES_ALLOWED) { return false; } File objFile = new File("t.tmp"); if (objFile.length() > MAX_FILE_SIZE) { // Do something here to manage the file size, such as renaming the file // You won't be able to easily remove an object from the file without a more // advanced file structure since you are writing arbitrary sized serialized // objects. You would need to do some kind of tagging of each entry or include // a record length before each one. Then you would have to scan and rebuild // a new file. You cannot easily just delete bytes earlier in the file without // even more advanced structures (like having an index, fixed size records and // free space lists, or even a database). } FileOutputStream fos = null; try { fos = new FileOutputStream(objFile, true); // Open for append ObjectOutputStream oos = new ObjectOutputStream(new BufferedOutputStream(fos)); oos.writeObject(eldest.getValue()); oos.close(); // Close the object stream to flush remaining generated data (if any). return true; } catch (IOException e) { // Log error here or.... throw new RuntimeException(e.getMessage(), e); // Convert to RuntimeException } finally { if (fos != null) { try { fos.close(); } catch (IOException e2) { // Log failure - no need to throw though } } } }孝箱捆讨