Shared DefaultConversionService instance for simple fallback purposes

Issue: SPR-14948
(cherry picked from commit 80931b2)

spring-core/src/main/java/org/springframework/core/convert/support/DefaultConversionService.java

@@ -52,6 +52,30 @@ public class DefaultConversionService extends GenericConversionService {
 	private static final boolean streamAvailable = ClassUtils.isPresent(
 			"java.util.stream.Stream", DefaultConversionService.class.getClassLoader());
 
+	private static volatile DefaultConversionService sharedInstance;
+
+
+	/**
+	 * Return a shared default {@code ConversionService} instance,
+	 * lazily building it once needed.
+	 * <p><b>NOTE:</b> We highly recommend constructing individual
+	 * {@code ConversionService} instances for customization purposes.
+	 * This accessor is only meant as a fallback for code paths which
+	 * need simple type coercion but cannot access a longer-lived
+	 * {@code ConversionService} instance any other way.
+	 * @return the shared {@code ConversionService} instance (never {@code null})
+	 * @since 4.3.5
+	 */
+	public static ConversionService getSharedInstance() {
+		if (sharedInstance == null) {
+			synchronized (DefaultConversionService.class) {
+				if (sharedInstance == null) {
+					sharedInstance = new DefaultConversionService();
+				}
+			}
+		}
+		return sharedInstance;
+	}
 
 
 	/**

spring-core/src/main/java/org/springframework/core/env/AbstractPropertyResolver.java

@@ -22,8 +22,11 @@ import java.util.Set;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 
+import org.springframework.core.convert.ConversionService;
 import org.springframework.core.convert.support.ConfigurableConversionService;
 import org.springframework.core.convert.support.DefaultConversionService;
+import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.PropertyPlaceholderHelper;
 import org.springframework.util.SystemPropertyUtils;
 
@@ -38,7 +41,7 @@ public abstract class AbstractPropertyResolver implements ConfigurablePropertyRe
 
 	protected final Log logger = LogFactory.getLog(getClass());
 
-	protected ConfigurableConversionService conversionService = new DefaultConversionService();
+	private volatile ConfigurableConversionService conversionService;
 
 	private PropertyPlaceholderHelper nonStrictHelper;
 
@@ -57,11 +60,21 @@ public abstract class AbstractPropertyResolver implements ConfigurablePropertyRe
 
 	@Override
 	public ConfigurableConversionService getConversionService() {
-		return this.conversionService;
+		// Need to provide an independent DefaultConversionService, not the
+		// shared DefaultConversionService used by PropertySourcesPropertyResolver.
+		if (this.conversionService == null) {
+			synchronized (this) {
+				if (this.conversionService == null) {
+					this.conversionService = new DefaultConversionService();
+				}
+			}
+		}
+		return conversionService;
 	}
 
 	@Override
 	public void setConversionService(ConfigurableConversionService conversionService) {
+		Assert.notNull(conversionService, "ConversionService must not be null");
 		this.conversionService = conversionService;
 	}
 
@@ -72,6 +85,7 @@ public abstract class AbstractPropertyResolver implements ConfigurablePropertyRe
 	 */
 	@Override
 	public void setPlaceholderPrefix(String placeholderPrefix) {
+		Assert.notNull(placeholderPrefix, "'placeholderPrefix' must not be null");
 		this.placeholderPrefix = placeholderPrefix;
 	}
 
@@ -82,6 +96,7 @@ public abstract class AbstractPropertyResolver implements ConfigurablePropertyRe
 	 */
 	@Override
 	public void setPlaceholderSuffix(String placeholderSuffix) {
+		Assert.notNull(placeholderSuffix, "'placeholderSuffix' must not be null");
 		this.placeholderSuffix = placeholderSuffix;
 	}
 
@@ -113,8 +128,10 @@ public abstract class AbstractPropertyResolver implements ConfigurablePropertyRe
 
 	@Override
 	public void setRequiredProperties(String... requiredProperties) {
-		for (String key : requiredProperties) {
-			this.requiredProperties.add(key);
+		if (requiredProperties != null) {
+			for (String key : requiredProperties) {
+				this.requiredProperties.add(key);
+			}
 		}
 	}
 
@@ -224,6 +241,31 @@ public abstract class AbstractPropertyResolver implements ConfigurablePropertyRe
 		});
 	}
 
+	/**
+	 * Convert the given value to the specified target type, if necessary.
+	 * @param value the original property value
+	 * @param targetType the specified target type for property retrieval
+	 * @return the converted value, or the original value if no conversion
+	 * is necessary
+	 * @since 4.3.5
+	 */
+	@SuppressWarnings("unchecked")
+	protected <T> T convertValueIfNecessary(Object value, Class<T> targetType) {
+		if (targetType == null) {
+			return (T) value;
+		}
+		ConversionService csToUse = this.conversionService;
+		if (csToUse == null) {
+			// Avoid initialization of shared DefaultConversionService if
+			// no standard type conversion is needed in the first place...
+			if (ClassUtils.isAssignableValue(targetType, value)) {
+				return (T) value;
+			}
+			csToUse = DefaultConversionService.getSharedInstance();
+		}
+		return csToUse.convert(value, targetType);
+	}
+
 
 	/**
 	 * Retrieve the specified property as a raw String,

spring-core/src/main/java/org/springframework/core/env/PropertySourcesPropertyResolver.java

@@ -83,7 +83,7 @@ public class PropertySourcesPropertyResolver extends AbstractPropertyResolver {
 						value = resolveNestedPlaceholders((String) value);
 					}
 					logKeyFound(key, propertySource, value);
-					return this.conversionService.convert(value, targetValueType);
+					return convertValueIfNecessary(value, targetValueType);
 				}
 			}
 		}