is final instance initiated and assigned before constructor? - java-7

Question

public static Properties prop;
private static UnionAuthProperties unionAuthProperties;
private UnionAuthProperties() {
try {
prop = PropertiesUtil.getPropertiesFromClassPath("unionauth.properties");
} catch (Exception e) {
LOG.error("{}",e);
}
}
public static synchronized UnionAuthProperties getInstance() {
if (unionAuthProperties == null) {
unionAuthProperties = new UnionAuthProperties(); #27
}
return unionAuthProperties;
}
private final String URL_REQUEST = prop.getProperty("url.request"); #32
The last statement URL_REQUEST causes:
threw exception
java.lang.NullPointerException
at UnionAuthProperties.<init>(UnionAuthProperties.java:32)
at UnionAuthProperties.getInstance(UnionAuthProperties.java:27)
at UnionAuthClient.<init>(UnionAuthClient.java:9)
based on my knowledge, instance no matter final or not are initiated after constructor [1] while the final ones have to be assigned before the end of constructor [2]. So why prop is NULL when URL_REQUEST is initialized?
EDIT: If right after super() and this(...) complete, instances are initialized, then the final instance REDIRECT_URI should be initialized to null or blank. However this prints REDIRECT_URI as REDIRECT_URI:
public class Test {
private static Properties prop;
public static void main(String[] args) {
Test a = new Test();
}
public Test() {
// The following code should run after private final String REDIRECT_URI;
try {
System.out.println();
} catch (Exception e) {}
REDIRECT_URI = "REDIRECT_URI";
System.out.println(REDIRECT_URI);
}
private final String REDIRECT_URI;
}
And I also tried to change the constructor like this:
private UnionAuthProperties() {
prop = new Properties();
}
still NPE.

Answer 1

http://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.5
Items 1-3 specify calling explicit or implicit variations of super() and this()
Item 4 says execute instance variable initializers from top to bottom.
That means all fields final and non-final that are not inside the constructor, just fields of the new instance are initialized now.
Item 5 says execute rest of the body of this constructor, where rest in your case starts with try { prop = .... Rest means "without super() or this()"
Hence prop.getProperty("url.request") is executed before prop is initialized in rest of the body of constructor.
I would move prop initialization to getInstance() function.
Here's the order in which your code executes (initializations and constructor code)
// 1. when class is loaded (because it is `static`) effectively same as public static Properties prop = null;
public static Properties prop;
// 2. when class is loaded; effectively same as unionAuthProperties = null
private static UnionAuthProperties unionAuthProperties;
private UnionAuthProperties() {
try {
// 5. executing constructor code
prop = PropertiesUtil.getPropertiesFromClassPath("unionauth.properties");
} catch (Exception e) {
LOG.error("{}",e);
}
}
public static synchronized UnionAuthProperties getInstance() {
if (unionAuthProperties == null) {
// 3. creating new instance (first only constructor called, assignment is later)
unionAuthProperties = new UnionAuthProperties(); #27
}
return unionAuthProperties;
}
// 4. `execute instance variable initializers` in top to bottom order
// prop is null yet
private final String URL_REQUEST = prop.getProperty("url.request");