Re: Questions about DataSourceElement

Mon, 05 Jun 2017 10:54:24 -0700 

Am 05.06.2017 um 19:43 schrieb Philippe Mouawad:

On Monday, June 5, 2017, Felix Schumacher <felix.schumac...@internetallee.de>
wrote:


Hi all,

while looking at the javadocs for DataSourceElement, I found a couple of
things, that look odd to me.

a) Instead of string concatenation with "+" for static strings, often a
Stringbuilder is used. So instead of:

String result = "shared: " + shared + " user: " + username + ...

The code looks like

StringBuilder builder = new StringBuilder(40);
builder.append("shared: ").append(shared)
     .append("user: ").append(username)
     ...
String result = builder.toString();

I think the compiler will generate the same code for the former and the
latter. To me the string concatenation looks cleaner.

Is there any reason to use the latter code for static strings?


If this part is intensive I am not sure perfs are not better with
StringBuilder

When you compile a class:
public class Test {

    public static String builder(String a, String b) {
        StringBuilder builder = new StringBuilder(10);
        builder.append("a: ").append(a)
            .append(" b: ").append(b);
        return builder.toString();
    }

    public static String concat(String a, String b) {
        return "a: " + a + " b: " + b;
    }

    public static void main(String[] args) {
        System.out.println("Builder: " + builder("a", "b"));
        System.out.println("Concatenation: " + concat("a", "b"));
    }
}

You get the (de-)compiled class [javap -c Test.class]:
Compiled from "Test.java"
public class Test {
  public Test();
    Code:
       0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V 
       4: return
public static java.lang.String builder(java.lang.String, java.lang.String); 
    Code:
0: new #2 // class java/lang/StringBuilder 
       3: dup
       4: bipush        10
6: invokespecial #3 // Method java/lang/StringBuilder."<init>":(I)V 
       9: astore_2
      10: aload_2
      11: ldc           #4                  // String a:
13: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 
      16: aload_0
17: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 
      20: ldc           #6                  // String  b:
22: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 
      25: aload_1
26: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 
      29: pop
      30: aload_2
31: invokevirtual #7 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 
      34: areturn
public static java.lang.String concat(java.lang.String, java.lang.String); 
    Code:
0: new #2 // class java/lang/StringBuilder 
       3: dup
4: invokespecial #8 // Method java/lang/StringBuilder."<init>":()V 
       7: ldc           #4                  // String a:
9: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 
      12: aload_0
13: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 
      16: ldc           #6                  // String  b:
18: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 
      21: aload_1
22: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 25: invokevirtual #7 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 
      28: areturn

  public static void main(java.lang.String[]);
    Code:
0: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream; 3: new #2 // class java/lang/StringBuilder 
       6: dup
7: invokespecial #8 // Method java/lang/StringBuilder."<init>":()V 
      10: ldc           #10                 // String Builder:
12: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 
      15: ldc           #11                 // String a
      17: ldc           #12                 // String b
19: invokestatic #13 // Method builder:(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String; 22: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 25: invokevirtual #7 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 28: invokevirtual #14 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 31: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream; 34: new #2 // class java/lang/StringBuilder 
      37: dup
38: invokespecial #8 // Method java/lang/StringBuilder."<init>":()V 
      41: ldc           #15                 // String Concatenation:
43: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 
      46: ldc           #11                 // String a
      48: ldc           #12                 // String b
50: invokestatic #16 // Method concat:(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String; 53: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 56: invokevirtual #7 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 59: invokevirtual #14 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 
      62: return
}
You will notice, that the methods concat() and builder() compile down to the same class instructions, with one difference. The initialisation of the StringBuilder is done with no argument in the concat() case and with our "10" in the builder() one. 




b) In the inner class  DataSourceComponentImpl there is some (really
minor) code duplication in getConnectionInfo and getConnection. But my real
concern is, that getConnection checks for a null BasicDataSource, while
getConnectionInfo doesn't.

What is the logic behind this?

A bug :)

So it wouldn't harm to check for null on getConnectionInfo, right?

Felix

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