java.security.egd 作用详解

public static UUID randomUUID() {
        SecureRandom ng =Holder.numberGenerator;
        byte[] randomBytes = new byte[16];
        ng.nextBytes(randomBytes);
        randomBytes[6] &= 0x0f;  /* clear version       */
        randomBytes[6]  |=0x40;  /* set to version 4     */
        randomBytes[8] &= 0x3f;  /* clear variant       */
        randomBytes[8]  |=0x80;  /* set to IETF variant  */
        return newUUID(randomBytes);
    }

public long nextLong() { 
        // it's okay that the bottom wordremains signed. 
        return ((long)(next(32)) << 32)+ next(32); 
    } 
    final protected int next(int numBits) { 
        int numBytes = (numBits+7)/8; 
        byte b[] = new byte[numBytes]; 
        int next = 0; 
        nextBytes(b);
        for (int i = 0; i < numBytes; i++) 
            next = (next << 8)+ (b[i] & 0xFF); 
        return next >>> (numBytes*8 -numBits); 
    }

synchronized public void nextBytes(byte[] bytes) { 
       secureRandomSpi.engineNextBytes(bytes); 
    }

Provider: Set SUN provider property[SecureRandom.NativePRNG/sun.security.provider.NativePRNG]

private void ensureBufferValid() throws IOException {
            ...
            readFully(urandomIn, urandomBuffer);
            ...
        }

abstract class SeedGenerator {
......
    static {
        String egdSource = SunEntries.getSeedSource();
        if (egdSource.equals(URL_DEV_RANDOM) || egdSource.equals(URL_DEV_URANDOM)) {
            try {
                instance = new NativeSeedGenerator();
                if (debug != null) {
                    debug.println("Using operating system seed generator");
                }
            } catch (IOException e) {
                if (debug != null) {
                    debug.println("Failed to use operating system seed "
                                  + "generator: " + e.toString());
                }
            }
        } else if (egdSource.length() != 0) {
            try {
                instance = new URLSeedGenerator(egdSource);
                if (debug != null) {
                    debug.println("Using URL seed generator reading from "
                                  + egdSource);
                }
            } catch (IOException e) {
                if (debug != null)
                    debug.println("Failed to create seed generator with "
                                  + egdSource + ": " + e.toString());
            }
        }
......
    }