RFR: 8267840: Improve URLStreamHandler.parseURL()

[Сергей Цыпанов]

There is an optimization opportunity for the widespread use-case when a resource is read from classpath using `getClass().getClassLoader().getResource()` or `getClass().getClassLoader().getResourceAsStream()`.

Pay attention to lines starting from 261. In case I run something like

var props = getClass().getClassLoader().getResource("/application.properties");

I get into the if-else block starting from 251 and here 'separator' variable is an empty String. In this case we can skip 'separator' from concatenation chain and use `String.concat()` as there are only two items concatenated.

In the opposite case `separator` variable is `"/"` and at the same time `ind` variable is `-1`. This means that expression `path.substring(0, ind + 1)` always returns an empty String and again can be excluded from concatenation chain allowing usage of `String.concat()` which allows to dodge utilization of `StringBuilder` (here `StringConcatFactory` is not available, see https://github.com/openjdk/jdk/pull/3627)

In the next else-block, starting from 274, again, `String.concat()` is applicable.

In another if-else block, starting from 277, when id is 0 again path.substring(0, ind) returns empty String making concatenation pointless and avoidable.

There are also some other minor clean-ups possible regarding constant conditions (lines 252 and 161).

The change allows to reduce significantly resource look-up costs for a very wide-spread case:

@State(Scope.Benchmark)
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@Fork(jvmArgsAppend = {"-Xms2g", "-Xmx2g"})
public class ClassGetResourceBenchmark {
    private final Class<?> clazz = getClass();

    @Benchmark
    public URL getResource() {
        return clazz.getResource("/application.properties");
    }
}

The change allows to reduce memory consumption significantly:

before

Benchmark                                                               Mode  Cnt     Score     Error   Units
ClassGetResourceBenchmark.getResource                                   avgt  100  1649,367 ±   5,904   ns/op
ClassGetResourceBenchmark.getResource:·gc.alloc.rate                    avgt  100   619,204 ±   2,413  MB/sec
ClassGetResourceBenchmark.getResource:·gc.alloc.rate.norm               avgt  100  1339,232 ±   4,909    B/op
ClassGetResourceBenchmark.getResource:·gc.churn.G1_Eden_Space           avgt  100   627,192 ±  74,972  MB/sec
ClassGetResourceBenchmark.getResource:·gc.churn.G1_Eden_Space.norm      avgt  100  1356,681 ± 162,354    B/op
ClassGetResourceBenchmark.getResource:·gc.churn.G1_Survivor_Space       avgt  100     0,119 ±   0,100  MB/sec
ClassGetResourceBenchmark.getResource:·gc.churn.G1_Survivor_Space.norm  avgt  100     0,257 ±   0,217    B/op
ClassGetResourceBenchmark.getResource:·gc.count                         avgt  100   128,000            counts
ClassGetResourceBenchmark.getResource:·gc.time                          avgt  100   227,000                ms

after

Benchmark                                                               Mode  Cnt     Score     Error   Units
ClassGetResourceBenchmark.getResource                                   avgt  100  1599,948 ±   4,115   ns/op
ClassGetResourceBenchmark.getResource:·gc.alloc.rate                    avgt  100   358,434 ±   0,922  MB/sec
ClassGetResourceBenchmark.getResource:·gc.alloc.rate.norm               avgt  100   752,016 ±   0,004    B/op
ClassGetResourceBenchmark.getResource:·gc.churn.G1_Eden_Space           avgt  100   342,778 ±  76,490  MB/sec
ClassGetResourceBenchmark.getResource:·gc.churn.G1_Eden_Space.norm      avgt  100   719,264 ± 160,513    B/op
ClassGetResourceBenchmark.getResource:·gc.churn.G1_Survivor_Space       avgt  100     0,008 ±   0,005  MB/sec
ClassGetResourceBenchmark.getResource:·gc.churn.G1_Survivor_Space.norm  avgt  100     0,017 ±   0,010    B/op
ClassGetResourceBenchmark.getResource:·gc.count                         avgt  100    70,000            counts
ClassGetResourceBenchmark.getResource:·gc.time                          avgt  100   151,000                ms

-------------

Commit messages:
 - 8267840: Improve URLStreamHandler.parseURL()

Changes: https://git.openjdk.java.net/jdk/pull/4526/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4526&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267840
  Stats: 25 lines in 1 file changed: 3 ins; 8 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4526.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4526/head:pull/4526

PR: https://git.openjdk.java.net/jdk/pull/4526