Java · Theory

Java FilePermission

Theory

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Java FilePermission

FilePermission is about reading or writing data outside the program — files, bytes, or characters. Byte streams (InputStream/OutputStream) handle raw bytes. Character streams (Reader/Writer) handle text.

For FilePermission, always close resources. Prefer try-with-resources so close happens even if an error occurs. Buffered streams are faster for many small reads.

FilePermission viva: serialization turns an object into bytes. transient fields are skipped. Don’t serialize secrets without a plan.

Let's take this on the board with one tiny Main class — no extra files. Java FilePermission — output: Hello File. StringReader → StringWriter is the same read/write pattern as a real file, and this Main always runs. Start from main, go line by line, and stop at each print. That output is the proof for FilePermission.

Exam tip

Always mention closing resources in I/O answers.

Example

import java.io.*;
public class Main {
  public static void main(String[] args) throws Exception {
    String text = "Hello File";
    try (StringReader in = new StringReader(text);
         StringWriter out = new StringWriter()) {
      int ch;
      while ((ch = in.read()) != -1) out.write(ch);
      System.out.println(out.toString());
    }
  }
}

Java FilePermission — output: Hello File. StringReader → StringWriter is the same read/write pattern as a real file, and this Main always runs.

Short notes

DefFilePermission — I/O = read/write outside the program (file, bytes, text).
RuleFilePermission — byte streams for bytes, Reader/Writer for text, close with try-with-resources.
RememberFilePermission — buffered streams are faster for many small reads.
TrapFilePermission — forgetting to close, or serializing secrets.

import java.io.*; public class Main { public static void main(String[] args) throws Exception { String text = "Hello File"; try (StringReader in = new StringReader(text); StringWriter out = new StringWriter()) { int ch; while ((ch = in.read()) != -1) out.write(ch); System.out.println(out.toString()); } } }

Java FilePermission

Example

Short notes

Questions

Java FilePermission

Example

Short notes

Questions