Use cache for base time unit strings (#4487)

* Use cache for base time unit strings

Closes gh-4352

* Polish "Use cache for base time unit strings"

---------

Co-authored-by: Jonatan Ivanov <[email protected]>

Author: izeye

micrometer-core/src/main/java/io/micrometer/core/instrument/MeterRegistry.java

@@ -36,6 +36,7 @@
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.*;
+import java.util.stream.Collectors;
 
 import static java.lang.String.format;
 import static java.util.Collections.emptyList;
@@ -59,6 +60,18 @@
  */
 public abstract class MeterRegistry {
 
+    // @formatter:off
+    private static final EnumMap<TimeUnit, String> BASE_TIME_UNIT_STRING_CACHE = Arrays.stream(TimeUnit.values())
+        .collect(
+            Collectors.toMap(
+                Function.identity(),
+                (timeUnit) -> timeUnit.toString().toLowerCase(),
+                (k, v) -> { throw new IllegalStateException("Duplicate keys should not exist."); },
+                () -> new EnumMap<>(TimeUnit.class)
+            )
+        );
+    // @formatter:on
+
     protected final Clock clock;
 
     private final Object meterMapLock = new Object();
@@ -280,7 +293,7 @@ protected String getConventionName(Meter.Id id) {
     protected abstract DistributionStatisticConfig defaultHistogramConfig();
 
     private String getBaseTimeUnitStr() {
-        return getBaseTimeUnit().toString().toLowerCase();
+        return BASE_TIME_UNIT_STRING_CACHE.get(getBaseTimeUnit());
     }
 
     /**

micrometer-core/src/test/java/io/micrometer/core/instrument/MeterRegistryTest.java

@@ -15,6 +15,7 @@
  */
 package io.micrometer.core.instrument;
 
+import io.micrometer.core.Issue;
 import io.micrometer.core.instrument.config.MeterFilter;
 import io.micrometer.core.instrument.config.MeterFilterReply;
 import io.micrometer.core.instrument.distribution.DistributionStatisticConfig;
@@ -205,4 +206,12 @@ void shouldNotLetRegisteringMetersTwice() {
             .hasNoCause();
     }
 
+    @Test
+    @Issue("#4352")
+    void baseUnitStringShouldBeCachedAndReturnTheSameInstance() {
+        Timer timer1 = registry.timer("test.timer1");
+        Timer timer2 = registry.timer("test.timer2");
+        assertThat(timer1.getId().getBaseUnit()).isSameAs(timer2.getId().getBaseUnit());
+    }
+
 }