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以前没有关注java的里面的SecurityManager这个类，最近看动态代理顺便记录下此类的使用。

其实很多时候都用到SecurityManager，只是平时没太关注而已，FileInputStream的构造方法：

public FileInputStream(File file) throws FileNotFoundException {
String name = (file != null ? file.getPath() : null);
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkRead(name);
}
if (name == null) {
throw new NullPointerException();
}
if (file.isInvalid()) {
throw new FileNotFoundException(“Invalid file path”);
}
fd = new FileDescriptor();
fd.incrementAndGetUseCount();
this.path = name;
open(name);
}

就有SecurityManager的用武之地。借用这个例子，有以下测试类：

import java.io.*;

/**
* Created by
*/
public class SecurityManageTest {

public static void main(String args[]) {    try {        System.setSecurityManager(new PasswordSecurityManager("a"));        System.setSecurityManager(new PasswordSecurityManager("a"));    } catch (SecurityException se) {        System.err.println("Exception already set!");    }    try {        System.out.println("start1");        BufferedReader fis = new BufferedReader(new FileReader("test.txt"));        System.out.println("start2");        BufferedWriter fos = new BufferedWriter(new FileWriter("test.txt"));        System.out.println("start3");        String inputString;        while ((inputString = fis.readLine()) != null) {            fos.write(inputString);            fos.write('\n');        }        fis.close();        fos.close();    } catch (IOException e) {        e.printStackTrace();    } catch (Exception e) {        e.printStackTrace();    }}private static class PasswordSecurityManager extends SecurityManager {    private String password;    PasswordSecurityManager(String password) {        super();        this.password = password;    }    private boolean accessOK() {        int c;        BufferedReader dis = new BufferedReader(new InputStreamReader(System.in));        String response;        System.out.println("What's the secret password?");        try {            response = dis.readLine();            if (response.equals(password))                return true;            else                return false;        } catch (IOException e) {            return false;        }    }    public void checkRead(FileDescriptor filedescriptor) {        if (!accessOK())            throw new SecurityException("no read permission!");    }    public void checkRead(String filename) {        if (!accessOK())            throw new SecurityException("no read permission!");    }    public void checkRead(String filename, Object executionContext) {        if (!accessOK())            throw new SecurityException("no read permission!");    }    public void checkWrite(FileDescriptor filedescriptor) {        if (!accessOK())            throw new SecurityException("no write permission!");    }    public void checkWrite(String filename) {        if (!accessOK())            throw new SecurityException("has no write permission!");    }}

}

运行结果如下：
start1
What’s the secret password?
a
start2
What’s the secret password?
a
start3