我无法检查您的 OpenJDK 具体版本，但我可以检查jdk6-b33。
SecureRandom 使用SeedGenerator 获取种子字节
public byte[] engineGenerateSeed(int numBytes) {
byte[] b = new byte[numBytes];
SeedGenerator.generateSeed(b);
return b;
}
SeedGenerator 从SunEntries 获取seedSource（字符串）
String egdSource = SunEntries.getSeedSource();
SunEntries首先尝试从系统属性java.security.egd获取源，如果找不到则尝试从java.security属性文件中获取属性securerandom.source，如果未找到该属性返回空白字符串。
// name of the *System* property, takes precedence over PROP_RNDSOURCE
private final static String PROP_EGD = "java.security.egd";
// name of the *Security* property
private final static String PROP_RNDSOURCE = "securerandom.source";
final static String URL_DEV_RANDOM = "file:/dev/random";
final static String URL_DEV_URANDOM = "file:/dev/urandom";
private static final String seedSource;
static {
seedSource = AccessController.doPrivileged(
new PrivilegedAction<String>() {
public String run() {
String egdSource = System.getProperty(PROP_EGD, "");
if (egdSource.length() != 0) {
return egdSource;
}
egdSource = Security.getProperty(PROP_RNDSOURCE);
if (egdSource == null) {
return "";
}
return egdSource;
}
});
}
SeedGenerator勾选这个值来初始化实例
// Static instance is created at link time
private static SeedGenerator instance;
private static final Debug debug = Debug.getInstance("provider");
final static String URL_DEV_RANDOM = SunEntries.URL_DEV_RANDOM;
final static String URL_DEV_URANDOM = SunEntries.URL_DEV_URANDOM;
// Static initializer to hook in selected or best performing generator
static {
String egdSource = SunEntries.getSeedSource();
// Try the URL specifying the source
// e.g. file:/dev/random
//
// The URL file:/dev/random or file:/dev/urandom is used to indicate
// the SeedGenerator using OS support, if available.
// On Windows, the causes MS CryptoAPI to be used.
// On Solaris and Linux, this is the identical to using
// URLSeedGenerator to read from /dev/random
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());
}
}
// Fall back to ThreadedSeedGenerator
if (instance == null) {
if (debug != null) {
debug.println("Using default threaded seed generator");
}
instance = new ThreadedSeedGenerator();
}
}
如果来源是
final static String URL_DEV_RANDOM = "file:/dev/random";
或
final static String URL_DEV_URANDOM = "file:/dev/urandom"
使用NativeSeedGenerator，在Windows 上尝试使用本机CryptoAPI，在Linux 上该类只是扩展了SeedGenerator.URLSeedGenerator
package sun.security.provider;
import java.io.IOException;
/**
* Native seed generator for Unix systems. Inherit everything from
* URLSeedGenerator.
*
*/
class NativeSeedGenerator extends SeedGenerator.URLSeedGenerator {
NativeSeedGenerator() throws IOException {
super();
}
}
并调用默认加载/dev/random的超类构造函数
URLSeedGenerator() throws IOException {
this(SeedGenerator.URL_DEV_RANDOM);
}
所以，OpenJDK 默认使用/dev/random，直到您没有在系统属性java.security.egd 或安全属性文件的属性securerandom.source 中设置其他值。
如果您想使用strace 查看读取结果，您可以更改命令行并添加trace=open,read 表达式
sudo strace -o a.strace -f -e trace=open,read java class
你可以看到这样的东西（我用 Oracle JDK 6 做了测试）
13225 open("/dev/random", O_RDONLY) = 8
13225 read(8, "@", 1) = 1
13225 read(3, "PK\3\4\n\0\0\0\0\0RyzB\36\320\267\325u\4\0\0u\4\0\0 \0\0\0", 30) = 30
....
....
如果您在启动过程中遇到延迟，为了加快启动速度，Tomcat Wiki 部分建议使用非阻塞熵源，例如 /dev/urandom
更多信息：https://wiki.apache.org/tomcat/HowTo/FasterStartUp#Entropy_Source
希望这会有所帮助。